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/auth/_default.py b/google/auth/_default.py
index f7e308f..7da77a2 100644
--- a/google/auth/_default.py
+++ b/google/auth/_default.py
@@ -23,8 +23,6 @@
 import os
 import warnings
 
-import six
-
 from google.auth import environment_vars
 from google.auth import exceptions
 import google.auth.transport._http_client
@@ -115,7 +113,7 @@
             new_exc = exceptions.DefaultCredentialsError(
                 "File {} is not a valid json file.".format(filename), caught_exc
             )
-            six.raise_from(new_exc, caught_exc)
+            raise new_exc from caught_exc
 
     # The type key should indicate that the file is either a service account
     # credentials file or an authorized user credentials file.
@@ -131,7 +129,7 @@
         except ValueError as caught_exc:
             msg = "Failed to load authorized user credentials from {}".format(filename)
             new_exc = exceptions.DefaultCredentialsError(msg, caught_exc)
-            six.raise_from(new_exc, caught_exc)
+            raise new_exc from caught_exc
         if quota_project_id:
             credentials = credentials.with_quota_project(quota_project_id)
         if not credentials.quota_project_id:
@@ -148,7 +146,7 @@
         except ValueError as caught_exc:
             msg = "Failed to load service account credentials from {}".format(filename)
             new_exc = exceptions.DefaultCredentialsError(msg, caught_exc)
-            six.raise_from(new_exc, caught_exc)
+            raise new_exc from caught_exc
         if quota_project_id:
             credentials = credentials.with_quota_project(quota_project_id)
         return credentials, info.get("project_id")