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/challenges.py b/google/oauth2/challenges.py
index d0b070e..7756a80 100644
--- a/google/oauth2/challenges.py
+++ b/google/oauth2/challenges.py
@@ -20,8 +20,6 @@
 import getpass
 import sys
 
-import six
-
 from google.auth import _helpers
 from google.auth import exceptions
 
@@ -44,8 +42,7 @@
     return getpass.getpass(text)
 
 
-@six.add_metaclass(abc.ABCMeta)
-class ReauthChallenge(object):
+class ReauthChallenge(object, metaclass=abc.ABCMeta):
     """Base class for reauth challenges."""
 
     @property