Fix DeprecationWarning about MutableMapping.

Fixes #484.

CHANGES.rst

@@ -4,7 +4,8 @@ Change History
 2.13.3 (unreleased)
 ===================
 
-- Nothing changed yet.
+- Fix DeprecationWarning about MutableMapping.
+  (`#484 <https://github.com/buildout/buildout/issues/484>_`)
 
 
 2.13.2 (2019-07-03)

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
 
 import zc.buildout.configparser
 import copy