try typing.MutableMapping for 3.7-3.8 support

Author: DenverCoder1

babel/localedata.py

@@ -19,9 +19,9 @@
 import sys
 import threading
 from collections import abc
-from collections.abc import Iterator, Mapping, MutableMapping
+from collections.abc import Iterator, Mapping
 from itertools import chain
-from typing import Any, TypeVar
+from typing import Any, TypeVar, MutableMapping
 
 _Key = TypeVar('_Key',)
 _Value = TypeVar('_Value')