fix: add back python 2.7 for gcloud usage only (#892)

* fix: add back python 2.7 for gcloud

* fix: fix setup and tests

* fix: add enum34 for python 2.7

* fix: add app engine app and fix noxfile

* fix: move test_app_engine.py

* fix: fix downscoped

* fix: fix downscoped

* fix: remove py2 from classifiers
diff --git a/google/auth/_cloud_sdk.py b/google/auth/_cloud_sdk.py
index 1f13ad4..40e6aec 100644
--- a/google/auth/_cloud_sdk.py
+++ b/google/auth/_cloud_sdk.py
@@ -18,6 +18,8 @@
 import os
 import subprocess
 
+import six
+
 from google.auth import environment_vars
 from google.auth import exceptions
 
@@ -154,4 +156,4 @@
         new_exc = exceptions.UserAccessTokenError(
             "Failed to obtain access token", caught_exc
         )
-        raise new_exc from caught_exc
+        six.raise_from(new_exc, caught_exc)
diff --git a/google/auth/_credentials_async.py b/google/auth/_credentials_async.py
index 760758d..d4d4e2c 100644
--- a/google/auth/_credentials_async.py
+++ b/google/auth/_credentials_async.py
@@ -18,10 +18,13 @@
 import abc
 import inspect
 
+import six
+
 from google.auth import credentials
 
 
-class Credentials(credentials.Credentials, metaclass=abc.ABCMeta):
+@six.add_metaclass(abc.ABCMeta)
+class Credentials(credentials.Credentials):
     """Async inherited credentials class from google.auth.credentials.
     The added functionality is the before_request call which requires
     async/await syntax.
@@ -81,7 +84,8 @@
     """
 
 
-class ReadOnlyScoped(credentials.ReadOnlyScoped, metaclass=abc.ABCMeta):
+@six.add_metaclass(abc.ABCMeta)
+class ReadOnlyScoped(credentials.ReadOnlyScoped):
     """Interface for credentials whose scopes can be queried.
 
     OAuth 2.0-based credentials allow limiting access using scopes as described
@@ -167,5 +171,6 @@
         return credentials
 
 
-class Signing(credentials.Signing, metaclass=abc.ABCMeta):
+@six.add_metaclass(abc.ABCMeta)
+class Signing(credentials.Signing):
     """Interface for credentials that can cryptographically sign messages."""
diff --git a/google/auth/_default.py b/google/auth/_default.py
index 8b0573b..4ae7c8c 100644
--- a/google/auth/_default.py
+++ b/google/auth/_default.py
@@ -23,6 +23,8 @@
 import os
 import warnings
 
+import six
+
 from google.auth import environment_vars
 from google.auth import exceptions
 import google.auth.transport._http_client
@@ -116,7 +118,7 @@
             new_exc = exceptions.DefaultCredentialsError(
                 "File {} is not a valid json file.".format(filename), caught_exc
             )
-            raise new_exc from caught_exc
+            six.raise_from(new_exc, caught_exc)
 
     # The type key should indicate that the file is either a service account
     # credentials file or an authorized user credentials file.
@@ -132,7 +134,7 @@
         except ValueError as caught_exc:
             msg = "Failed to load authorized user credentials from {}".format(filename)
             new_exc = exceptions.DefaultCredentialsError(msg, caught_exc)
-            raise new_exc from caught_exc
+            six.raise_from(new_exc, caught_exc)
         if quota_project_id:
             credentials = credentials.with_quota_project(quota_project_id)
         if not credentials.quota_project_id:
@@ -149,7 +151,7 @@
         except ValueError as caught_exc:
             msg = "Failed to load service account credentials from {}".format(filename)
             new_exc = exceptions.DefaultCredentialsError(msg, caught_exc)
-            raise new_exc from caught_exc
+            six.raise_from(new_exc, caught_exc)
         if quota_project_id:
             credentials = credentials.with_quota_project(quota_project_id)
         return credentials, info.get("project_id")
diff --git a/google/auth/_default_async.py b/google/auth/_default_async.py
index 3fa125b..fb277c5 100644
--- a/google/auth/_default_async.py
+++ b/google/auth/_default_async.py
@@ -21,6 +21,8 @@
 import json
 import os
 
+import six
+
 from google.auth import _default
 from google.auth import environment_vars
 from google.auth import exceptions
@@ -61,7 +63,7 @@
             new_exc = exceptions.DefaultCredentialsError(
                 "File {} is not a valid json file.".format(filename), caught_exc
             )
-            raise new_exc from caught_exc
+            six.raise_from(new_exc, caught_exc)
 
     # The type key should indicate that the file is either a service account
     # credentials file or an authorized user credentials file.
@@ -77,7 +79,7 @@
         except ValueError as caught_exc:
             msg = "Failed to load authorized user credentials from {}".format(filename)
             new_exc = exceptions.DefaultCredentialsError(msg, caught_exc)
-            raise new_exc from caught_exc
+            six.raise_from(new_exc, caught_exc)
         if quota_project_id:
             credentials = credentials.with_quota_project(quota_project_id)
         if not credentials.quota_project_id:
@@ -94,7 +96,7 @@
         except ValueError as caught_exc:
             msg = "Failed to load service account credentials from {}".format(filename)
             new_exc = exceptions.DefaultCredentialsError(msg, caught_exc)
-            raise new_exc from caught_exc
+            six.raise_from(new_exc, caught_exc)
         return credentials, info.get("project_id")
 
     else:
diff --git a/google/auth/_helpers.py b/google/auth/_helpers.py
index 55adf5b..b239fcd 100644
--- a/google/auth/_helpers.py
+++ b/google/auth/_helpers.py
@@ -17,7 +17,9 @@
 import base64
 import calendar
 import datetime
-import urllib
+
+import six
+from six.moves import urllib
 
 
 # Token server doesn't provide a new a token when doing refresh unless the
@@ -85,6 +87,9 @@
 def to_bytes(value, encoding="utf-8"):
     """Converts a string value to bytes, if necessary.
 
+    Unfortunately, ``six.b`` is insufficient for this task since in
+    Python 2 because it does not modify ``unicode`` objects.
+
     Args:
         value (Union[str, bytes]): The value to be converted.
         encoding (str): The encoding to use to convert unicode to bytes.
@@ -97,8 +102,8 @@
     Raises:
         ValueError: If the value could not be converted to bytes.
     """
-    result = value.encode(encoding) if isinstance(value, str) else value
-    if isinstance(result, bytes):
+    result = value.encode(encoding) if isinstance(value, six.text_type) else value
+    if isinstance(result, six.binary_type):
         return result
     else:
         raise ValueError("{0!r} could not be converted to bytes".format(value))
@@ -117,8 +122,8 @@
     Raises:
         ValueError: If the value could not be converted to unicode.
     """
-    result = value.decode("utf-8") if isinstance(value, bytes) else value
-    if isinstance(result, str):
+    result = value.decode("utf-8") if isinstance(value, six.binary_type) else value
+    if isinstance(result, six.text_type):
         return result
     else:
         raise ValueError("{0!r} could not be converted to unicode".format(value))
@@ -160,7 +165,7 @@
     query_params.update(params)
     # Remove any values specified in remove.
     query_params = {
-        key: value for key, value in query_params.items() if key not in remove
+        key: value for key, value in six.iteritems(query_params) if key not in remove
     }
     # Re-encoded the query string.
     new_query = urllib.parse.urlencode(query_params, doseq=True)
diff --git a/google/auth/_oauth2client.py b/google/auth/_oauth2client.py
index 3512e1d..95a9876 100644
--- a/google/auth/_oauth2client.py
+++ b/google/auth/_oauth2client.py
@@ -21,6 +21,8 @@
 
 from __future__ import absolute_import
 
+import six
+
 from google.auth import _helpers
 import google.auth.app_engine
 import google.auth.compute_engine
@@ -32,7 +34,7 @@
     import oauth2client.contrib.gce
     import oauth2client.service_account
 except ImportError as caught_exc:
-    raise ImportError("oauth2client is not installed.") from caught_exc
+    six.raise_from(ImportError("oauth2client is not installed."), caught_exc)
 
 try:
     import oauth2client.contrib.appengine  # pytype: disable=import-error
@@ -164,4 +166,4 @@
         return _CLASS_CONVERSION_MAP[credentials_class](credentials)
     except KeyError as caught_exc:
         new_exc = ValueError(_CONVERT_ERROR_TMPL.format(credentials_class))
-        raise new_exc from caught_exc
+        six.raise_from(new_exc, caught_exc)
diff --git a/google/auth/_service_account_info.py b/google/auth/_service_account_info.py
index 54a40e9..3d340c7 100644
--- a/google/auth/_service_account_info.py
+++ b/google/auth/_service_account_info.py
@@ -17,6 +17,8 @@
 import io
 import json
 
+import six
+
 from google.auth import crypt
 
 
@@ -41,7 +43,7 @@
     """
     keys_needed = set(require if require is not None else [])
 
-    missing = keys_needed.difference(data)
+    missing = keys_needed.difference(six.iterkeys(data))
 
     if missing:
         raise ValueError(
diff --git a/google/auth/aws.py b/google/auth/aws.py
index c8deee7..925b1dd 100644
--- a/google/auth/aws.py
+++ b/google/auth/aws.py
@@ -39,13 +39,20 @@
 
 import hashlib
 import hmac
-import http.client
 import io
 import json
 import os
+import posixpath
 import re
-import urllib
-from urllib.parse import urljoin
+
+try:
+    from urllib.parse import urljoin
+# Python 2.7 compatibility
+except ImportError:  # pragma: NO COVER
+    from urlparse import urljoin
+
+from six.moves import http_client
+from six.moves import urllib
 
 from google.auth import _helpers
 from google.auth import environment_vars
@@ -116,7 +123,9 @@
         # Normalize the URL path. This is needed for the canonical_uri.
         # os.path.normpath can't be used since it normalizes "/" paths
         # to "\\" in Windows OS.
-        normalized_uri = urllib.parse.urlparse(urljoin(url, uri.path))
+        normalized_uri = urllib.parse.urlparse(
+            urljoin(url, posixpath.normpath(uri.path))
+        )
         # Validate provided URL.
         if not uri.hostname or uri.scheme != "https":
             raise ValueError("Invalid AWS service URL")
@@ -631,7 +640,7 @@
             else response.data
         )
 
-        if response.status != http.client.OK:
+        if response.status != http_client.OK:
             raise exceptions.RefreshError(
                 "Unable to retrieve AWS security credentials", response_body
             )
@@ -670,7 +679,7 @@
             else response.data
         )
 
-        if response.status != http.client.OK:
+        if response.status != http_client.OK:
             raise exceptions.RefreshError(
                 "Unable to retrieve AWS role name", response_body
             )
diff --git a/google/auth/compute_engine/_metadata.py b/google/auth/compute_engine/_metadata.py
index af0d849..9db7bea 100644
--- a/google/auth/compute_engine/_metadata.py
+++ b/google/auth/compute_engine/_metadata.py
@@ -18,11 +18,13 @@
 """
 
 import datetime
-import http.client
 import json
 import logging
 import os
-from urllib import parse as urlparse
+
+import six
+from six.moves import http_client
+from six.moves.urllib import parse as urlparse
 
 from google.auth import _helpers
 from google.auth import environment_vars
@@ -89,7 +91,7 @@
 
             metadata_flavor = response.headers.get(_METADATA_FLAVOR_HEADER)
             return (
-                response.status == http.client.OK
+                response.status == http_client.OK
                 and metadata_flavor == _METADATA_FLAVOR_VALUE
             )
 
@@ -163,7 +165,7 @@
             "metadata service. Compute Engine Metadata server unavailable".format(url)
         )
 
-    if response.status == http.client.OK:
+    if response.status == http_client.OK:
         content = _helpers.from_bytes(response.data)
         if response.headers["content-type"] == "application/json":
             try:
@@ -173,7 +175,7 @@
                     "Received invalid JSON from the Google Compute Engine"
                     "metadata service: {:.20}".format(content)
                 )
-                raise new_exc from caught_exc
+                six.raise_from(new_exc, caught_exc)
         else:
             return content
     else:
diff --git a/google/auth/compute_engine/credentials.py b/google/auth/compute_engine/credentials.py
index cb4e0f0..b39ac50 100644
--- a/google/auth/compute_engine/credentials.py
+++ b/google/auth/compute_engine/credentials.py
@@ -21,6 +21,8 @@
 
 import datetime
 
+import six
+
 from google.auth import _helpers
 from google.auth import credentials
 from google.auth import exceptions
@@ -112,7 +114,7 @@
             )
         except exceptions.TransportError as caught_exc:
             new_exc = exceptions.RefreshError(caught_exc)
-            raise new_exc from caught_exc
+            six.raise_from(new_exc, caught_exc)
 
     @property
     def service_account_email(self):
@@ -350,7 +352,7 @@
             id_token = _metadata.get(request, path, params=params)
         except exceptions.TransportError as caught_exc:
             new_exc = exceptions.RefreshError(caught_exc)
-            raise new_exc from caught_exc
+            six.raise_from(new_exc, caught_exc)
 
         _, payload, _, _ = jwt._unverified_decode(id_token)
         return id_token, datetime.datetime.fromtimestamp(payload["exp"])
diff --git a/google/auth/credentials.py b/google/auth/credentials.py
index 8d9974c..ec21a27 100644
--- a/google/auth/credentials.py
+++ b/google/auth/credentials.py
@@ -17,10 +17,13 @@
 
 import abc
 
+import six
+
 from google.auth import _helpers
 
 
-class Credentials(object, metaclass=abc.ABCMeta):
+@six.add_metaclass(abc.ABCMeta)
+class Credentials(object):
     """Base class for all credentials.
 
     All credentials have a :attr:`token` that is used for authentication and
@@ -184,7 +187,8 @@
         """Anonymous credentials do nothing to the request."""
 
 
-class ReadOnlyScoped(object, metaclass=abc.ABCMeta):
+@six.add_metaclass(abc.ABCMeta)
+class ReadOnlyScoped(object):
     """Interface for credentials whose scopes can be queried.
 
     OAuth 2.0-based credentials allow limiting access using scopes as described
@@ -325,7 +329,8 @@
         return credentials
 
 
-class Signing(object, metaclass=abc.ABCMeta):
+@six.add_metaclass(abc.ABCMeta)
+class Signing(object):
     """Interface for credentials that can cryptographically sign messages."""
 
     @abc.abstractmethod
diff --git a/google/auth/crypt/__init__.py b/google/auth/crypt/__init__.py
index 97e9d81..15ac950 100644
--- a/google/auth/crypt/__init__.py
+++ b/google/auth/crypt/__init__.py
@@ -37,6 +37,8 @@
 version is at least 1.4.0.
 """
 
+import six
+
 from google.auth.crypt import base
 from google.auth.crypt import rsa
 
@@ -88,7 +90,7 @@
     Returns:
         bool: True if the signature is valid, otherwise False.
     """
-    if isinstance(certs, (str, bytes)):
+    if isinstance(certs, (six.text_type, six.binary_type)):
         certs = [certs]
 
     for cert in certs:
diff --git a/google/auth/crypt/_python_rsa.py b/google/auth/crypt/_python_rsa.py
index 1c4a9da..ec30dd0 100644
--- a/google/auth/crypt/_python_rsa.py
+++ b/google/auth/crypt/_python_rsa.py
@@ -21,13 +21,12 @@
 
 from __future__ import absolute_import
 
-import io
-
 from pyasn1.codec.der import decoder
 from pyasn1_modules import pem
 from pyasn1_modules.rfc2459 import Certificate
 from pyasn1_modules.rfc5208 import PrivateKeyInfo
 import rsa
+import six
 
 from google.auth import _helpers
 from google.auth.crypt import base
@@ -53,9 +52,9 @@
     """
     num_bits = len(bit_list)
     byte_vals = bytearray()
-    for start in range(0, num_bits, 8):
+    for start in six.moves.xrange(0, num_bits, 8):
         curr_bits = bit_list[start : start + 8]
-        char_val = sum(val * digit for val, digit in zip(_POW2, curr_bits))
+        char_val = sum(val * digit for val, digit in six.moves.zip(_POW2, curr_bits))
         byte_vals.append(char_val)
     return bytes(byte_vals)
 
@@ -153,7 +152,7 @@
         """
         key = _helpers.from_bytes(key)  # PEM expects str in Python 3
         marker_id, key_bytes = pem.readPemBlocksFromFile(
-            io.StringIO(key), _PKCS1_MARKER, _PKCS8_MARKER
+            six.StringIO(key), _PKCS1_MARKER, _PKCS8_MARKER
         )
 
         # Key is in pkcs1 format.
diff --git a/google/auth/crypt/base.py b/google/auth/crypt/base.py
index 0bda9c3..c98d5bf 100644
--- a/google/auth/crypt/base.py
+++ b/google/auth/crypt/base.py
@@ -18,12 +18,15 @@
 import io
 import json
 
+import six
+
 
 _JSON_FILE_PRIVATE_KEY = "private_key"
 _JSON_FILE_PRIVATE_KEY_ID = "private_key_id"
 
 
-class Verifier(object, metaclass=abc.ABCMeta):
+@six.add_metaclass(abc.ABCMeta)
+class Verifier(object):
     """Abstract base class for crytographic signature verifiers."""
 
     @abc.abstractmethod
@@ -43,7 +46,8 @@
         raise NotImplementedError("Verify must be implemented")
 
 
-class Signer(object, metaclass=abc.ABCMeta):
+@six.add_metaclass(abc.ABCMeta)
+class Signer(object):
     """Abstract base class for cryptographic signers."""
 
     @abc.abstractproperty
@@ -66,7 +70,8 @@
         raise NotImplementedError("Sign must be implemented")
 
 
-class FromServiceAccountMixin(object, metaclass=abc.ABCMeta):
+@six.add_metaclass(abc.ABCMeta)
+class FromServiceAccountMixin(object):
     """Mix-in to enable factory constructors for a Signer."""
 
     @abc.abstractmethod
diff --git a/google/auth/crypt/es256.py b/google/auth/crypt/es256.py
index 71dcbfc..c6d6176 100644
--- a/google/auth/crypt/es256.py
+++ b/google/auth/crypt/es256.py
@@ -15,6 +15,7 @@
 """ECDSA (ES256) verifier and signer that use the ``cryptography`` library.
 """
 
+from cryptography import utils
 import cryptography.exceptions
 from cryptography.hazmat import backends
 from cryptography.hazmat.primitives import hashes
@@ -52,8 +53,8 @@
         sig_bytes = _helpers.to_bytes(signature)
         if len(sig_bytes) != 64:
             return False
-        r = int.from_bytes(sig_bytes[:32], byteorder="big")
-        s = int.from_bytes(sig_bytes[32:], byteorder="big")
+        r = utils.int_from_bytes(sig_bytes[:32], byteorder="big")
+        s = utils.int_from_bytes(sig_bytes[32:], byteorder="big")
         asn1_sig = encode_dss_signature(r, s)
 
         message = _helpers.to_bytes(message)
@@ -120,7 +121,7 @@
 
         # Convert ASN1 encoded signature to (r||s) raw signature.
         (r, s) = decode_dss_signature(asn1_signature)
-        return r.to_bytes(32, byteorder="big") + s.to_bytes(32, byteorder="big")
+        return utils.int_to_bytes(r, 32) + utils.int_to_bytes(s, 32)
 
     @classmethod
     def from_string(cls, key, key_id=None):
diff --git a/google/auth/downscoped.py b/google/auth/downscoped.py
index 96a4e65..a1d7b6e 100644
--- a/google/auth/downscoped.py
+++ b/google/auth/downscoped.py
@@ -50,6 +50,8 @@
 
 import datetime
 
+import six
+
 from google.auth import _helpers
 from google.auth import credentials
 from google.oauth2 import sts
@@ -221,7 +223,7 @@
         Raises:
             TypeError: If the value is not a string.
         """
-        if not isinstance(value, str):
+        if not isinstance(value, six.string_types):
             raise TypeError("The provided available_resource is not a string.")
         self._available_resource = value
 
@@ -247,7 +249,7 @@
             ValueError: If the value is not valid.
         """
         for available_permission in value:
-            if not isinstance(available_permission, str):
+            if not isinstance(available_permission, six.string_types):
                 raise TypeError(
                     "Provided available_permissions are not a list of strings."
                 )
@@ -350,7 +352,7 @@
         Raises:
             TypeError: If the value is not of type string.
         """
-        if not isinstance(value, str):
+        if not isinstance(value, six.string_types):
             raise TypeError("The provided expression is not a string.")
         self._expression = value
 
@@ -373,7 +375,7 @@
         Raises:
             TypeError: If the value is not of type string or None.
         """
-        if not isinstance(value, str) and value is not None:
+        if not isinstance(value, six.string_types) and value is not None:
             raise TypeError("The provided title is not a string or None.")
         self._title = value
 
@@ -396,7 +398,7 @@
         Raises:
             TypeError: If the value is not of type string or None.
         """
-        if not isinstance(value, str) and value is not None:
+        if not isinstance(value, six.string_types) and value is not None:
             raise TypeError("The provided description is not a string or None.")
         self._description = value
 
diff --git a/google/auth/external_account.py b/google/auth/external_account.py
index f588981..cbd0baf 100644
--- a/google/auth/external_account.py
+++ b/google/auth/external_account.py
@@ -33,6 +33,8 @@
 import json
 import re
 
+import six
+
 from google.auth import _helpers
 from google.auth import credentials
 from google.auth import exceptions
@@ -50,9 +52,8 @@
 _CLOUD_RESOURCE_MANAGER = "https://cloudresourcemanager.googleapis.com/v1/projects/"
 
 
-class Credentials(
-    credentials.Scoped, credentials.CredentialsWithQuotaProject, metaclass=abc.ABCMeta
-):
+@six.add_metaclass(abc.ABCMeta)
+class Credentials(credentials.Scoped, credentials.CredentialsWithQuotaProject):
     """Base class for all external account credentials.
 
     This is used to instantiate Credentials for exchanging external account
diff --git a/google/auth/iam.py b/google/auth/iam.py
index 277f4b7..5d63dc5 100644
--- a/google/auth/iam.py
+++ b/google/auth/iam.py
@@ -20,9 +20,10 @@
 """
 
 import base64
-import http.client
 import json
 
+from six.moves import http_client
+
 from google.auth import _helpers
 from google.auth import crypt
 from google.auth import exceptions
@@ -76,7 +77,7 @@
         self._credentials.before_request(self._request, method, url, headers)
         response = self._request(url=url, method=method, body=body, headers=headers)
 
-        if response.status != http.client.OK:
+        if response.status != http_client.OK:
             raise exceptions.TransportError(
                 "Error calling the IAM signBlob API: {}".format(response.data)
             )
diff --git a/google/auth/identity_pool.py b/google/auth/identity_pool.py
index 901fd62..fb33d77 100644
--- a/google/auth/identity_pool.py
+++ b/google/auth/identity_pool.py
@@ -33,7 +33,11 @@
 access tokens.
 """
 
-from collections.abc import Mapping
+try:
+    from collections.abc import Mapping
+# Python 2.7 compatibility
+except ImportError:  # pragma: NO COVER
+    from collections import Mapping
 import io
 import json
 import os
diff --git a/google/auth/impersonated_credentials.py b/google/auth/impersonated_credentials.py
index 2704bfd..b8a6c49 100644
--- a/google/auth/impersonated_credentials.py
+++ b/google/auth/impersonated_credentials.py
@@ -28,9 +28,11 @@
 import base64
 import copy
 from datetime import datetime
-import http.client
 import json
 
+import six
+from six.moves import http_client
+
 from google.auth import _helpers
 from google.auth import credentials
 from google.auth import exceptions
@@ -98,7 +100,7 @@
         else response.data
     )
 
-    if response.status != http.client.OK:
+    if response.status != http_client.OK:
         exceptions.RefreshError(_REFRESH_ERROR, response_body)
 
     try:
@@ -115,7 +117,7 @@
             ),
             response_body,
         )
-        raise new_exc from caught_exc
+        six.raise_from(new_exc, caught_exc)
 
 
 class Credentials(credentials.CredentialsWithQuotaProject, credentials.Signing):
diff --git a/google/auth/jwt.py b/google/auth/jwt.py
index bb9ffae..d565595 100644
--- a/google/auth/jwt.py
+++ b/google/auth/jwt.py
@@ -40,13 +40,18 @@
 
 """
 
-from collections.abc import Mapping
+try:
+    from collections.abc import Mapping
+# Python 2.7 compatibility
+except ImportError:  # pragma: NO COVER
+    from collections import Mapping
 import copy
 import datetime
 import json
-import urllib
 
 import cachetools
+import six
+from six.moves import urllib
 
 from google.auth import _helpers
 from google.auth import _service_account_info
@@ -118,7 +123,7 @@
         return json.loads(section_bytes.decode("utf-8"))
     except ValueError as caught_exc:
         new_exc = ValueError("Can't parse segment: {0}".format(section_bytes))
-        raise new_exc from caught_exc
+        six.raise_from(new_exc, caught_exc)
 
 
 def _unverified_decode(token):
@@ -244,16 +249,19 @@
 
     try:
         verifier_cls = _ALGORITHM_TO_VERIFIER_CLASS[key_alg]
-    except KeyError as caught_exc:
+    except KeyError as exc:
         if key_alg in _CRYPTOGRAPHY_BASED_ALGORITHMS:
-            msg = (
-                "The key algorithm {} requires the cryptography package "
-                "to be installed."
+            six.raise_from(
+                ValueError(
+                    "The key algorithm {} requires the cryptography package "
+                    "to be installed.".format(key_alg)
+                ),
+                exc,
             )
         else:
-            msg = "Unsupported signature algorithm {}"
-        new_exc = ValueError(msg.format(key_alg))
-        raise new_exc from caught_exc
+            six.raise_from(
+                ValueError("Unsupported signature algorithm {}".format(key_alg)), exc
+            )
 
     # If certs is specified as a dictionary of key IDs to certificates, then
     # use the certificate identified by the key ID in the token header.
diff --git a/google/auth/transport/__init__.py b/google/auth/transport/__init__.py
index d1b035d..374e7b4 100644
--- a/google/auth/transport/__init__.py
+++ b/google/auth/transport/__init__.py
@@ -25,9 +25,11 @@
 """
 
 import abc
-import http.client
 
-DEFAULT_REFRESH_STATUS_CODES = (http.client.UNAUTHORIZED,)
+import six
+from six.moves import http_client
+
+DEFAULT_REFRESH_STATUS_CODES = (http_client.UNAUTHORIZED,)
 """Sequence[int]:  Which HTTP status code indicate that credentials should be
 refreshed and a request should be retried.
 """
@@ -36,7 +38,8 @@
 """int: How many times to refresh the credentials and retry a request."""
 
 
-class Response(object, metaclass=abc.ABCMeta):
+@six.add_metaclass(abc.ABCMeta)
+class Response(object):
     """HTTP Response data."""
 
     @abc.abstractproperty
@@ -55,7 +58,8 @@
         raise NotImplementedError("data must be implemented.")
 
 
-class Request(object, metaclass=abc.ABCMeta):
+@six.add_metaclass(abc.ABCMeta)
+class Request(object):
     """Interface for a callable that makes HTTP requests.
 
     Specific transport implementations should provide an implementation of
diff --git a/google/auth/transport/_aiohttp_requests.py b/google/auth/transport/_aiohttp_requests.py
index ee94043..ab7dfef 100644
--- a/google/auth/transport/_aiohttp_requests.py
+++ b/google/auth/transport/_aiohttp_requests.py
@@ -24,6 +24,7 @@
 import functools
 
 import aiohttp
+import six
 import urllib3
 
 from google.auth import exceptions
@@ -190,11 +191,11 @@
 
         except aiohttp.ClientError as caught_exc:
             new_exc = exceptions.TransportError(caught_exc)
-            raise new_exc from caught_exc
+            six.raise_from(new_exc, caught_exc)
 
         except asyncio.TimeoutError as caught_exc:
             new_exc = exceptions.TransportError(caught_exc)
-            raise new_exc from caught_exc
+            six.raise_from(new_exc, caught_exc)
 
 
 class AuthorizedSession(aiohttp.ClientSession):
diff --git a/google/auth/transport/_http_client.py b/google/auth/transport/_http_client.py
index 679087f..c153763 100644
--- a/google/auth/transport/_http_client.py
+++ b/google/auth/transport/_http_client.py
@@ -14,10 +14,12 @@
 
 """Transport adapter for http.client, for internal use only."""
 
-import http.client
 import logging
 import socket
-import urllib
+
+import six
+from six.moves import http_client
+from six.moves import urllib
 
 from google.auth import exceptions
 from google.auth import transport
@@ -96,7 +98,7 @@
                 "was specified".format(parts.scheme)
             )
 
-        connection = http.client.HTTPConnection(parts.netloc, timeout=timeout)
+        connection = http_client.HTTPConnection(parts.netloc, timeout=timeout)
 
         try:
             _LOGGER.debug("Making request: %s %s", method, url)
@@ -105,9 +107,9 @@
             response = connection.getresponse()
             return Response(response)
 
-        except (http.client.HTTPException, socket.error) as caught_exc:
+        except (http_client.HTTPException, socket.error) as caught_exc:
             new_exc = exceptions.TransportError(caught_exc)
-            raise new_exc from caught_exc
+            six.raise_from(new_exc, caught_exc)
 
         finally:
             connection.close()
diff --git a/google/auth/transport/_mtls_helper.py b/google/auth/transport/_mtls_helper.py
index 1b9b9c2..4dccb10 100644
--- a/google/auth/transport/_mtls_helper.py
+++ b/google/auth/transport/_mtls_helper.py
@@ -20,6 +20,8 @@
 import re
 import subprocess
 
+import six
+
 from google.auth import exceptions
 
 CONTEXT_AWARE_METADATA_PATH = "~/.secureConnect/context_aware_metadata.json"
@@ -80,7 +82,7 @@
             metadata = json.load(f)
     except ValueError as caught_exc:
         new_exc = exceptions.ClientCertError(caught_exc)
-        raise new_exc from caught_exc
+        six.raise_from(new_exc, caught_exc)
 
     return metadata
 
@@ -108,7 +110,7 @@
         stdout, stderr = process.communicate()
     except OSError as caught_exc:
         new_exc = exceptions.ClientCertError(caught_exc)
-        raise new_exc from caught_exc
+        six.raise_from(new_exc, caught_exc)
 
     # Check cert provider command execution error.
     if process.returncode != 0:
diff --git a/google/auth/transport/grpc.py b/google/auth/transport/grpc.py
index 160dc94..c47cb3d 100644
--- a/google/auth/transport/grpc.py
+++ b/google/auth/transport/grpc.py
@@ -19,6 +19,8 @@
 import logging
 import os
 
+import six
+
 from google.auth import environment_vars
 from google.auth import exceptions
 from google.auth.transport import _mtls_helper
@@ -27,11 +29,13 @@
 try:
     import grpc
 except ImportError as caught_exc:  # pragma: NO COVER
-    new_exc = ImportError(
-        "gRPC is not installed, please install the grpcio package "
-        "to use the gRPC transport."
+    six.raise_from(
+        ImportError(
+            "gRPC is not installed, please install the grpcio package "
+            "to use the gRPC transport."
+        ),
+        caught_exc,
     )
-    raise new_exc from caught_exc
 
 _LOGGER = logging.getLogger(__name__)
 
@@ -84,7 +88,7 @@
             self._request, context.method_name, context.service_url, headers
         )
 
-        return list(headers.items())
+        return list(six.iteritems(headers))
 
     def __call__(self, context, callback):
         """Passes authorization metadata into the given callback.
@@ -333,7 +337,7 @@
                 )
             except exceptions.ClientCertError as caught_exc:
                 new_exc = exceptions.MutualTLSChannelError(caught_exc)
-                raise new_exc from caught_exc
+                six.raise_from(new_exc, caught_exc)
         else:
             self._ssl_credentials = grpc.ssl_channel_credentials()
 
diff --git a/google/auth/transport/mtls.py b/google/auth/transport/mtls.py
index c570761..b40bfbe 100644
--- a/google/auth/transport/mtls.py
+++ b/google/auth/transport/mtls.py
@@ -14,6 +14,8 @@
 
 """Utilites for mutual TLS."""
 
+import six
+
 from google.auth import exceptions
 from google.auth.transport import _mtls_helper
 
@@ -51,7 +53,7 @@
             _, cert_bytes, key_bytes = _mtls_helper.get_client_cert_and_key()
         except (OSError, RuntimeError, ValueError) as caught_exc:
             new_exc = exceptions.MutualTLSChannelError(caught_exc)
-            raise new_exc from caught_exc
+            six.raise_from(new_exc, caught_exc)
 
         return cert_bytes, key_bytes
 
@@ -96,7 +98,7 @@
                 key_file.write(key_bytes)
         except (exceptions.ClientCertError, OSError) as caught_exc:
             new_exc = exceptions.MutualTLSChannelError(caught_exc)
-            raise new_exc from caught_exc
+            six.raise_from(new_exc, caught_exc)
 
         return cert_path, key_path, passphrase_bytes
 
diff --git a/google/auth/transport/requests.py b/google/auth/transport/requests.py
index 2cb6942..817176b 100644
--- a/google/auth/transport/requests.py
+++ b/google/auth/transport/requests.py
@@ -25,16 +25,21 @@
 try:
     import requests
 except ImportError as caught_exc:  # pragma: NO COVER
-    new_exc = ImportError(
-        "The requests library is not installed, please install the "
-        "requests package to use the requests transport."
+    import six
+
+    six.raise_from(
+        ImportError(
+            "The requests library is not installed, please install the "
+            "requests package to use the requests transport."
+        ),
+        caught_exc,
     )
-    raise new_exc from caught_exc
 import requests.adapters  # pylint: disable=ungrouped-imports
 import requests.exceptions  # pylint: disable=ungrouped-imports
 from requests.packages.urllib3.util.ssl_ import (
     create_urllib3_context,
 )  # pylint: disable=ungrouped-imports
+import six  # pylint: disable=ungrouped-imports
 
 from google.auth import environment_vars
 from google.auth import exceptions
@@ -181,7 +186,7 @@
             return _Response(response)
         except requests.exceptions.RequestException as caught_exc:
             new_exc = exceptions.TransportError(caught_exc)
-            raise new_exc from caught_exc
+            six.raise_from(new_exc, caught_exc)
 
 
 class _MutualTlsAdapter(requests.adapters.HTTPAdapter):
@@ -391,7 +396,7 @@
             import OpenSSL
         except ImportError as caught_exc:
             new_exc = exceptions.MutualTLSChannelError(caught_exc)
-            raise new_exc from caught_exc
+            six.raise_from(new_exc, caught_exc)
 
         try:
             (
@@ -411,7 +416,7 @@
             OpenSSL.crypto.Error,
         ) as caught_exc:
             new_exc = exceptions.MutualTLSChannelError(caught_exc)
-            raise new_exc from caught_exc
+            six.raise_from(new_exc, caught_exc)
 
     def request(
         self,
diff --git a/google/auth/transport/urllib3.py b/google/auth/transport/urllib3.py
index aa7188c..6a2504d 100644
--- a/google/auth/transport/urllib3.py
+++ b/google/auth/transport/urllib3.py
@@ -34,11 +34,16 @@
 try:
     import urllib3
 except ImportError as caught_exc:  # pragma: NO COVER
-    new_exc = ImportError(
-        "The urllib3 library is not installed, please install the "
-        "urllib3 package to use the urllib3 transport."
+    import six
+
+    six.raise_from(
+        ImportError(
+            "The urllib3 library is not installed, please install the "
+            "urllib3 package to use the urllib3 transport."
+        ),
+        caught_exc,
     )
-    raise new_exc from caught_exc
+import six
 import urllib3.exceptions  # pylint: disable=ungrouped-imports
 
 from google.auth import environment_vars
@@ -137,7 +142,7 @@
             return _Response(response)
         except urllib3.exceptions.HTTPError as caught_exc:
             new_exc = exceptions.TransportError(caught_exc)
-            raise new_exc from caught_exc
+            six.raise_from(new_exc, caught_exc)
 
 
 def _make_default_http():
@@ -329,7 +334,7 @@
             import OpenSSL
         except ImportError as caught_exc:
             new_exc = exceptions.MutualTLSChannelError(caught_exc)
-            raise new_exc from caught_exc
+            six.raise_from(new_exc, caught_exc)
 
         try:
             found_cert_key, cert, key = transport._mtls_helper.get_client_cert_and_key(
@@ -346,7 +351,7 @@
             OpenSSL.crypto.Error,
         ) as caught_exc:
             new_exc = exceptions.MutualTLSChannelError(caught_exc)
-            raise new_exc from caught_exc
+            six.raise_from(new_exc, caught_exc)
 
         if self._has_user_provided_http:
             self._has_user_provided_http = False
diff --git a/google/oauth2/_client.py b/google/oauth2/_client.py
index f819371..2f4e847 100644
--- a/google/oauth2/_client.py
+++ b/google/oauth2/_client.py
@@ -24,9 +24,11 @@
 """
 
 import datetime
-import http.client
 import json
-import urllib
+
+import six
+from six.moves import http_client
+from six.moves import urllib
 
 from google.auth import _helpers
 from google.auth import exceptions
@@ -118,7 +120,7 @@
         )
         response_data = json.loads(response_body)
 
-        if response.status == http.client.OK:
+        if response.status == http_client.OK:
             break
         else:
             error_desc = response_data.get("error_description") or ""
@@ -129,9 +131,9 @@
             ):
                 retry += 1
                 continue
-            return response.status == http.client.OK, response_data
+            return response.status == http_client.OK, response_data
 
-    return response.status == http.client.OK, response_data
+    return response.status == http_client.OK, response_data
 
 
 def _token_endpoint_request(
@@ -194,7 +196,7 @@
         access_token = response_data["access_token"]
     except KeyError as caught_exc:
         new_exc = exceptions.RefreshError("No access token in response.", response_data)
-        raise new_exc from caught_exc
+        six.raise_from(new_exc, caught_exc)
 
     expiry = _parse_expiry(response_data)
 
@@ -234,7 +236,7 @@
         id_token = response_data["id_token"]
     except KeyError as caught_exc:
         new_exc = exceptions.RefreshError("No ID token in response.", response_data)
-        raise new_exc from caught_exc
+        six.raise_from(new_exc, caught_exc)
 
     payload = jwt.decode(id_token, verify=False)
     expiry = datetime.datetime.utcfromtimestamp(payload["exp"])
@@ -263,7 +265,7 @@
         access_token = response_data["access_token"]
     except KeyError as caught_exc:
         new_exc = exceptions.RefreshError("No access token in response.", response_data)
-        raise new_exc from caught_exc
+        six.raise_from(new_exc, caught_exc)
 
     refresh_token = response_data.get("refresh_token", refresh_token)
     expiry = _parse_expiry(response_data)
diff --git a/google/oauth2/_client_async.py b/google/oauth2/_client_async.py
index 8849023..cf51211 100644
--- a/google/oauth2/_client_async.py
+++ b/google/oauth2/_client_async.py
@@ -24,9 +24,11 @@
 """
 
 import datetime
-import http.client
 import json
-import urllib
+
+import six
+from six.moves import http_client
+from six.moves import urllib
 
 from google.auth import exceptions
 from google.auth import jwt
@@ -83,7 +85,7 @@
 
         response_data = json.loads(response_body)
 
-        if response.status == http.client.OK:
+        if response.status == http_client.OK:
             break
         else:
             error_desc = response_data.get("error_description") or ""
@@ -94,9 +96,9 @@
             ):
                 retry += 1
                 continue
-            return response.status == http.client.OK, response_data
+            return response.status == http_client.OK, response_data
 
-    return response.status == http.client.OK, response_data
+    return response.status == http_client.OK, response_data
 
 
 async def _token_endpoint_request(
@@ -159,7 +161,7 @@
         access_token = response_data["access_token"]
     except KeyError as caught_exc:
         new_exc = exceptions.RefreshError("No access token in response.", response_data)
-        raise new_exc from caught_exc
+        six.raise_from(new_exc, caught_exc)
 
     expiry = client._parse_expiry(response_data)
 
@@ -199,7 +201,7 @@
         id_token = response_data["id_token"]
     except KeyError as caught_exc:
         new_exc = exceptions.RefreshError("No ID token in response.", response_data)
-        raise new_exc from caught_exc
+        six.raise_from(new_exc, caught_exc)
 
     payload = jwt.decode(id_token, verify=False)
     expiry = datetime.datetime.utcfromtimestamp(payload["exp"])
diff --git a/google/oauth2/_id_token_async.py b/google/oauth2/_id_token_async.py
index a4a526d..31fcbc6 100644
--- a/google/oauth2/_id_token_async.py
+++ b/google/oauth2/_id_token_async.py
@@ -58,10 +58,12 @@
 .. _CacheControl: https://cachecontrol.readthedocs.io
 """
 
-import http.client
 import json
 import os
 
+import six
+from six.moves import http_client
+
 from google.auth import environment_vars
 from google.auth import exceptions
 from google.auth import jwt
@@ -86,7 +88,7 @@
     """
     response = await request(certs_url, method="GET")
 
-    if response.status != http.client.OK:
+    if response.status != http_client.OK:
         raise exceptions.TransportError(
             "Could not fetch certificates at {}".format(certs_url)
         )
@@ -241,7 +243,7 @@
                 "GOOGLE_APPLICATION_CREDENTIALS is not valid service account credentials.",
                 caught_exc,
             )
-            raise new_exc from caught_exc
+            six.raise_from(new_exc, caught_exc)
 
     # 2. Try to fetch ID token from metada server if it exists. The code works
     # for GAE and Cloud Run metadata server as well.
diff --git a/google/oauth2/_reauth_async.py b/google/oauth2/_reauth_async.py
index f74f50b..0276ddd 100644
--- a/google/oauth2/_reauth_async.py
+++ b/google/oauth2/_reauth_async.py
@@ -34,6 +34,8 @@
 
 import sys
 
+from six.moves import range
+
 from google.auth import exceptions
 from google.oauth2 import _client
 from google.oauth2 import _client_async
diff --git a/google/oauth2/challenges.py b/google/oauth2/challenges.py
index 0baff62..95e76cb 100644
--- a/google/oauth2/challenges.py
+++ b/google/oauth2/challenges.py
@@ -20,6 +20,8 @@
 import getpass
 import sys
 
+import six
+
 from google.auth import _helpers
 from google.auth import exceptions
 
@@ -45,7 +47,8 @@
     return getpass.getpass(text)
 
 
-class ReauthChallenge(object, metaclass=abc.ABCMeta):
+@six.add_metaclass(abc.ABCMeta)
+class ReauthChallenge(object):
     """Base class for reauth challenges."""
 
     @property
diff --git a/google/oauth2/credentials.py b/google/oauth2/credentials.py
index 6d34edf..9b59f8c 100644
--- a/google/oauth2/credentials.py
+++ b/google/oauth2/credentials.py
@@ -35,6 +35,8 @@
 import io
 import json
 
+import six
+
 from google.auth import _cloud_sdk
 from google.auth import _helpers
 from google.auth import credentials
@@ -262,7 +264,7 @@
         if self._refresh_token is None and self.refresh_handler:
             token, expiry = self.refresh_handler(request, scopes=scopes)
             # Validate returned data.
-            if not isinstance(token, str):
+            if not isinstance(token, six.string_types):
                 raise exceptions.RefreshError(
                     "The refresh_handler returned token is not a string."
                 )
@@ -344,7 +346,7 @@
             ValueError: If the info is not in the expected format.
         """
         keys_needed = set(("refresh_token", "client_id", "client_secret"))
-        missing = keys_needed.difference(info)
+        missing = keys_needed.difference(six.iterkeys(info))
 
         if missing:
             raise ValueError(
@@ -364,7 +366,7 @@
         # process scopes, which needs to be a seq
         if scopes is None and "scopes" in info:
             scopes = info.get("scopes")
-            if isinstance(scopes, str):
+            if isinstance(scopes, six.string_types):
                 scopes = scopes.split(" ")
 
         return cls(
diff --git a/google/oauth2/id_token.py b/google/oauth2/id_token.py
index 25492ca..8d0f85a 100644
--- a/google/oauth2/id_token.py
+++ b/google/oauth2/id_token.py
@@ -55,10 +55,12 @@
 .. _CacheControl: https://cachecontrol.readthedocs.io
 """
 
-import http.client
 import json
 import os
 
+import six
+from six.moves import http_client
+
 from google.auth import environment_vars
 from google.auth import exceptions
 from google.auth import jwt
@@ -95,7 +97,7 @@
     """
     response = request(certs_url, method="GET")
 
-    if response.status != http.client.OK:
+    if response.status != http_client.OK:
         raise exceptions.TransportError(
             "Could not fetch certificates at {}".format(certs_url)
         )
@@ -240,7 +242,7 @@
                 "GOOGLE_APPLICATION_CREDENTIALS is not valid service account credentials.",
                 caught_exc,
             )
-            raise new_exc from caught_exc
+            six.raise_from(new_exc, caught_exc)
 
     # 2. Try to fetch ID token from metada server if it exists. The code
     # works for GAE and Cloud Run metadata server as well.
diff --git a/google/oauth2/reauth.py b/google/oauth2/reauth.py
index 1e496d1..cbf1d7f 100644
--- a/google/oauth2/reauth.py
+++ b/google/oauth2/reauth.py
@@ -34,6 +34,8 @@
 
 import sys
 
+from six.moves import range
+
 from google.auth import exceptions
 from google.oauth2 import _client
 from google.oauth2 import challenges
diff --git a/google/oauth2/sts.py b/google/oauth2/sts.py
index 9f2d68a..ae3c014 100644
--- a/google/oauth2/sts.py
+++ b/google/oauth2/sts.py
@@ -31,9 +31,10 @@
 .. _rfc8693 section 2.2.1: https://tools.ietf.org/html/rfc8693#section-2.2.1
 """
 
-import http.client
 import json
-import urllib
+
+from six.moves import http_client
+from six.moves import urllib
 
 from google.oauth2 import utils
 
@@ -145,7 +146,7 @@
         )
 
         # If non-200 response received, translate to OAuthError exception.
-        if response.status != http.client.OK:
+        if response.status != http_client.OK:
             utils.handle_error_response(response_body)
 
         response_data = json.loads(response_body)
diff --git a/google/oauth2/utils.py b/google/oauth2/utils.py
index c57833d..593f032 100644
--- a/google/oauth2/utils.py
+++ b/google/oauth2/utils.py
@@ -45,6 +45,8 @@
 import enum
 import json
 
+import six
+
 from google.auth import exceptions
 
 
@@ -75,7 +77,8 @@
         self.client_secret = client_secret
 
 
-class OAuthClientAuthHandler(object, metaclass=abc.ABCMeta):
+@six.add_metaclass(abc.ABCMeta)
+class OAuthClientAuthHandler(object):
     """Abstract class for handling client authentication in OAuth-based
     operations.
     """