Fix DeprecationWarning on collections import

flask/_compat.py

@@ -28,6 +28,7 @@ if not PY2:
 
     from inspect import getfullargspec as getargspec
     from io import StringIO
+    import collections.abc as collections_abc
 
     def reraise(tp, value, tb=None):
         if value.__traceback__ is not tb:
@@ -47,6 +48,7 @@ else:
 
     from inspect import getargspec
     from cStringIO import StringIO
+    import collections as collections_abc
 
     exec('def reraise(tp, value, tb=None):\n raise tp, value, tb')
 

flask/sessions.py

@@ -11,17 +11,17 @@
 
 import hashlib
 import warnings
-from collections import MutableMapping
 from datetime import datetime
 
 from itsdangerous import BadSignature, URLSafeTimedSerializer
 from werkzeug.datastructures import CallbackDict
 
+from flask._compat import collections_abc
 from flask.helpers import is_ip, total_seconds
 from flask.json.tag import TaggedJSONSerializer
 
 
-class SessionMixin(MutableMapping):
+class SessionMixin(collections_abc.MutableMapping):
     """Expands a basic dictionary with session attributes."""
 
     @property