Import stdlib ABCs from 'collections.abc' rather than 'collections'. (#6451)

On Python 2.7, fall back to 'collections'.

Closes #6450.

Author: tseaver

packages/google-cloud-core/google/cloud/iam.py

@@ -18,6 +18,10 @@
 """
 
 import collections
+try:
+    from collections import abc as collections_abc
+except ImportError:  # Python 2.7
+    import collections as collections_abc
 import warnings
 
 # Generic IAM roles
@@ -35,7 +39,7 @@
 Assigning to '{}' is deprecated.  Replace with 'policy[{}] = members."""
 
 
-class Policy(collections.MutableMapping):
+class Policy(collections_abc.MutableMapping):
     """IAM Policy
 
     See
@@ -239,4 +243,4 @@ def to_api_repr(self):
         return resource
 
 
-collections.MutableMapping.register(Policy)
+collections_abc.MutableMapping.register(Policy)