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/credentials.py b/google/oauth2/credentials.py
index 158249e..98fd71b 100644
--- a/google/oauth2/credentials.py
+++ b/google/oauth2/credentials.py
@@ -35,8 +35,6 @@
 import io
 import json
 
-import six
-
 from google.auth import _cloud_sdk
 from google.auth import _helpers
 from google.auth import credentials
@@ -336,7 +334,7 @@
             ValueError: If the info is not in the expected format.
         """
         keys_needed = set(("refresh_token", "client_id", "client_secret"))
-        missing = keys_needed.difference(six.iterkeys(info))
+        missing = keys_needed.difference(info)
 
         if missing:
             raise ValueError(