Resolve DeprecationWarning

Resolves "DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working" - see wagtail#5484

wagtail/utils/l18n/translation.py

@@ -2,7 +2,7 @@
 import gettext
 import bisect
 from locale import getdefaultlocale
-from collections import MutableMapping
+from collections.abc import MutableMapping
 from copy import copy, deepcopy
 
 import six