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_iam.py b/tests/test_iam.py
index e9eca58..bc71225 100644
--- a/tests/test_iam.py
+++ b/tests/test_iam.py
@@ -14,11 +14,11 @@
 
 import base64
 import datetime
-import http.client
 import json
 
 import mock
 import pytest
+from six.moves import http_client
 
 from google.auth import _helpers
 from google.auth import exceptions
@@ -81,7 +81,7 @@
     def test_sign_bytes(self):
         signature = b"DEADBEEF"
         encoded_signature = base64.b64encode(signature).decode("utf-8")
-        request = make_request(http.client.OK, data={"signedBlob": encoded_signature})
+        request = make_request(http_client.OK, data={"signedBlob": encoded_signature})
         credentials = make_credentials()
 
         signer = iam.Signer(request, credentials, mock.sentinel.service_account_email)
@@ -93,7 +93,7 @@
         assert kwargs["headers"]["Content-Type"] == "application/json"
 
     def test_sign_bytes_failure(self):
-        request = make_request(http.client.UNAUTHORIZED)
+        request = make_request(http_client.UNAUTHORIZED)
         credentials = make_credentials()
 
         signer = iam.Signer(request, credentials, mock.sentinel.service_account_email)