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/reauth.py b/google/oauth2/reauth.py
index 1e496d1..cbf1d7f 100644
--- a/google/oauth2/reauth.py
+++ b/google/oauth2/reauth.py
@@ -34,6 +34,8 @@
 
 import sys
 
+from six.moves import range
+
 from google.auth import exceptions
 from google.oauth2 import _client
 from google.oauth2 import challenges