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/sts.py b/google/oauth2/sts.py
index 9f2d68a..ae3c014 100644
--- a/google/oauth2/sts.py
+++ b/google/oauth2/sts.py
@@ -31,9 +31,10 @@
 .. _rfc8693 section 2.2.1: https://tools.ietf.org/html/rfc8693#section-2.2.1
 """
 
-import http.client
 import json
-import urllib
+
+from six.moves import http_client
+from six.moves import urllib
 
 from google.oauth2 import utils
 
@@ -145,7 +146,7 @@
         )
 
         # If non-200 response received, translate to OAuthError exception.
-        if response.status != http.client.OK:
+        if response.status != http_client.OK:
             utils.handle_error_response(response_body)
 
         response_data = json.loads(response_body)