Fix DeprecationWarning about MutableMapping.

Fixes #484.

(cherry picked from commit 5aa3e5bd)

src/zc/buildout/buildout.py

@@ -23,9 +23,9 @@ except ImportError:
     from md5 import md5
 
 try:
-    from UserDict import DictMixin
+    from collections.abc import MutableMapping as DictMixin
 except ImportError:
-    from collections import MutableMapping as DictMixin
+    from UserDict import DictMixin
 
 try:
     from cStringIO import StringIO