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/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