Alias and use compat version of collections.abc.MutableMapping

Since the MutableMapping class will no longer be available to import
directly from the collections module in Python 3.8, we should create an
alias for it in the compat module and use that instead.

rest_framework/compat.py

@@ -12,10 +12,10 @@ from django.views.generic import View
 
 try:
     # Python 3
-    from collections.abc import Mapping   # noqa
+    from collections.abc import Mapping, MutableMapping   # noqa
 except ImportError:
     # Python 2.7
-    from collections import Mapping   # noqa
+    from collections import Mapping, MutableMapping   # noqa
 
 try:
     # Python 3

rest_framework/utils/serializer_helpers.py

@@ -1,11 +1,10 @@
 from __future__ import unicode_literals
 
-import collections
 from collections import OrderedDict
 
 from django.utils.encoding import force_text
 
-from rest_framework.compat import unicode_to_repr
+from rest_framework.compat import MutableMapping, unicode_to_repr
 from rest_framework.utils import json
 
 
@@ -130,7 +129,7 @@ class NestedBoundField(BoundField):
         return self.__class__(self._field, values, self.errors, self._prefix)
 
 
-class BindingDict(collections.MutableMapping):
+class BindingDict(MutableMapping):
     """
     This dict-like object is used to store fields on a serializer.