Fix DeprecationWarning in tests when accessing collections.abc classes via collections

[tirkarthi]

This is a follow up PR on #6268 to fix similar cases in tests as noted in #6268 (comment)

Thanks for the library :)

[carltongibson]

If you can fix the lint failure I'll take this, but I'd rather wait until we're running against 3.8 to be honest. I'm not even seeing these warnings running the test suite on 3.7 with -Wall.

[tirkarthi]

I have fixed the linter errors. I too couldn't see the warnings but I am not sure if pytest is doing something with that. Using python3.7 -Wall on master shows me the warnings as below :

(pydev-venv)  > ~/django-rest-framework on master   python3.7 -Wall tests/test_serializer.py
tests/test_serializer.py:8: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
  from collections import Mapping
Traceback (most recent call last):
  File "tests/test_serializer.py", line 10, in <module>
    import pytest
ModuleNotFoundError: No module named 'pytest'
(pydev-venv)  > ~/django-rest-framework on master   python3.7 -Wall tests/test_renderers.py
tests/test_renderers.py:5: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
  from collections import MutableMapping, OrderedDict
Traceback (most recent call last):
  File "tests/test_renderers.py", line 7, in <module>
    import pytest
ModuleNotFoundError: No module named 'pytest'

[carltongibson]

OK, super. Thanks for the update.