fix!: drop support for Python 2.7 (#778)

Drop use of 'six' wrapper library.

Drop 'u"' prefixes.

Drop support for app_engine 'classic' mode (Python 2.7-only).

Release-As: 2.0.0b1

Closes #777.
diff --git a/google/oauth2/_client.py b/google/oauth2/_client.py
index 2f4e847..f819371 100644
--- a/google/oauth2/_client.py
+++ b/google/oauth2/_client.py
@@ -24,11 +24,9 @@
 """
 
 import datetime
+import http.client
 import json
-
-import six
-from six.moves import http_client
-from six.moves import urllib
+import urllib
 
 from google.auth import _helpers
 from google.auth import exceptions
@@ -120,7 +118,7 @@
         )
         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 ""
@@ -131,9 +129,9 @@
             ):
                 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(
@@ -196,7 +194,7 @@
         access_token = response_data["access_token"]
     except KeyError as caught_exc:
         new_exc = exceptions.RefreshError("No access token in response.", response_data)
-        six.raise_from(new_exc, caught_exc)
+        raise new_exc from caught_exc
 
     expiry = _parse_expiry(response_data)
 
@@ -236,7 +234,7 @@
         id_token = response_data["id_token"]
     except KeyError as caught_exc:
         new_exc = exceptions.RefreshError("No ID token in response.", response_data)
-        six.raise_from(new_exc, caught_exc)
+        raise new_exc from caught_exc
 
     payload = jwt.decode(id_token, verify=False)
     expiry = datetime.datetime.utcfromtimestamp(payload["exp"])
@@ -265,7 +263,7 @@
         access_token = response_data["access_token"]
     except KeyError as caught_exc:
         new_exc = exceptions.RefreshError("No access token in response.", response_data)
-        six.raise_from(new_exc, caught_exc)
+        raise new_exc from caught_exc
 
     refresh_token = response_data.get("refresh_token", refresh_token)
     expiry = _parse_expiry(response_data)