Fix DeprecationWarning: collections.MutableMapping

DeprecationWarning: Using or importing the ABCs from 'collections'
instead of from 'collections.abc' is deprecated, and in 3.8 it will stop
working

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 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