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/test__oauth2client.py b/tests/test__oauth2client.py
index aa06ece..6b1112b 100644
--- a/tests/test__oauth2client.py
+++ b/tests/test__oauth2client.py
@@ -13,7 +13,6 @@
 # limitations under the License.
 
 import datetime
-import importlib
 import os
 import sys
 
@@ -22,6 +21,7 @@
 import oauth2client.contrib.gce
 import oauth2client.service_account
 import pytest
+from six.moves import reload_module
 
 from google.auth import _oauth2client
 
@@ -152,19 +152,19 @@
 @pytest.fixture
 def reset__oauth2client_module():
     """Reloads the _oauth2client module after a test."""
-    importlib.reload(_oauth2client)
+    reload_module(_oauth2client)
 
 
 def test_import_has_app_engine(
     mock_oauth2client_gae_imports, reset__oauth2client_module
 ):
-    importlib.reload(_oauth2client)
+    reload_module(_oauth2client)
     assert _oauth2client._HAS_APPENGINE
 
 
 def test_import_without_oauth2client(monkeypatch, reset__oauth2client_module):
     monkeypatch.setitem(sys.modules, "oauth2client", None)
     with pytest.raises(ImportError) as excinfo:
-        importlib.reload(_oauth2client)
+        reload_module(_oauth2client)
 
     assert excinfo.match("oauth2client")