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/tests/oauth2/test__client.py b/tests/oauth2/test__client.py
index 690a87b..54686df 100644
--- a/tests/oauth2/test__client.py
+++ b/tests/oauth2/test__client.py
@@ -13,13 +13,14 @@
 # limitations under the License.
 
 import datetime
-import http.client
 import json
 import os
-import urllib
 
 import mock
 import pytest
+import six
+from six.moves import http_client
+from six.moves import urllib
 
 from google.auth import _helpers
 from google.auth import crypt
@@ -74,7 +75,7 @@
     assert _client._parse_expiry({}) is None
 
 
-def make_request(response_data, status=http.client.OK):
+def make_request(response_data, status=http_client.OK):
     response = mock.create_autospec(transport.Response, instance=True)
     response.status = status
     response.data = json.dumps(response_data).encode("utf-8")
@@ -129,7 +130,7 @@
 
 
 def test__token_endpoint_request_error():
-    request = make_request({}, status=http.client.BAD_REQUEST)
+    request = make_request({}, status=http_client.BAD_REQUEST)
 
     with pytest.raises(exceptions.RefreshError):
         _client._token_endpoint_request(request, "http://example.com", {})
@@ -137,7 +138,7 @@
 
 def test__token_endpoint_request_internal_failure_error():
     request = make_request(
-        {"error_description": "internal_failure"}, status=http.client.BAD_REQUEST
+        {"error_description": "internal_failure"}, status=http_client.BAD_REQUEST
     )
 
     with pytest.raises(exceptions.RefreshError):
@@ -146,7 +147,7 @@
         )
 
     request = make_request(
-        {"error": "internal_failure"}, status=http.client.BAD_REQUEST
+        {"error": "internal_failure"}, status=http_client.BAD_REQUEST
     )
 
     with pytest.raises(exceptions.RefreshError):
@@ -159,7 +160,7 @@
     request_body = request.call_args[1]["body"].decode("utf-8")
     request_params = urllib.parse.parse_qs(request_body)
 
-    for key, value in params.items():
+    for key, value in six.iteritems(params):
         assert request_params[key][0] == value