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/auth/external_account.py b/google/auth/external_account.py
index f588981..cbd0baf 100644
--- a/google/auth/external_account.py
+++ b/google/auth/external_account.py
@@ -33,6 +33,8 @@
 import json
 import re
 
+import six
+
 from google.auth import _helpers
 from google.auth import credentials
 from google.auth import exceptions
@@ -50,9 +52,8 @@
 _CLOUD_RESOURCE_MANAGER = "https://cloudresourcemanager.googleapis.com/v1/projects/"
 
 
-class Credentials(
-    credentials.Scoped, credentials.CredentialsWithQuotaProject, metaclass=abc.ABCMeta
-):
+@six.add_metaclass(abc.ABCMeta)
+class Credentials(credentials.Scoped, credentials.CredentialsWithQuotaProject):
     """Base class for all external account credentials.
 
     This is used to instantiate Credentials for exchanging external account