squash! Revert "Fix DeprecationWarning: collections.MutableMapping"

This reverts commit 40d511e.

 > Traceback:
 > /usr/lib/python2.7/dist-packages/six.py:709: in exec_
 >     exec("""exec _code_ in _globs_, _locs_""")
 > tests/test_interfaces.py:13: in <module>
 >     from univention.config_registry.interfaces import _Iface, Interfaces, forgiving, cmp_alnum  # noqa E402
 > python/univention/config_registry/__init__.py:33: in <module>
 >     from univention.config_registry.backend import StrictModeException, SCOPE, ConfigRegistry  # noqa F401
 > python/univention/config_registry/backend.py:39: in <module>
 >     from collections.abc import MutableMapping
 > E   ImportError: No module named abc

There is no six.moves.collections.abc yet:
<benjaminp/six#155>

base/univention-config-registry/python/univention/config_registry/backend.py

@@ -36,7 +36,10 @@
 import re
 import errno
 import time
-from collections.abc import MutableMapping
+try:
+	from collections.abc import MutableMapping  # Python 3.3+
+except ImportError:
+	from collections import MutableMapping
 import six
 try:
 	from typing import overload, Any, Dict, IO, Iterator, List, NoReturn, Optional, Set, Tuple, Type, TypeVar, Union  # noqa F401