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/identity_pool.py b/google/auth/identity_pool.py
index 901fd62..fb33d77 100644
--- a/google/auth/identity_pool.py
+++ b/google/auth/identity_pool.py
@@ -33,7 +33,11 @@
 access tokens.
 """
 
-from collections.abc import Mapping
+try:
+    from collections.abc import Mapping
+# Python 2.7 compatibility
+except ImportError:  # pragma: NO COVER
+    from collections import Mapping
 import io
 import json
 import os