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/utils.py b/google/oauth2/utils.py
index c57833d..593f032 100644
--- a/google/oauth2/utils.py
+++ b/google/oauth2/utils.py
@@ -45,6 +45,8 @@
 import enum
 import json
 
+import six
+
 from google.auth import exceptions
 
 
@@ -75,7 +77,8 @@
         self.client_secret = client_secret
 
 
-class OAuthClientAuthHandler(object, metaclass=abc.ABCMeta):
+@six.add_metaclass(abc.ABCMeta)
+class OAuthClientAuthHandler(object):
     """Abstract class for handling client authentication in OAuth-based
     operations.
     """