Fix upstream changes from DRF-3.10

[tfranzel]

DRF has dropped python 2 support with version 3.10 and Django has dropped it in master. If this lib wants to keep compatibility it needs to pull out six support into requirements.txt as it cannot import six from new rest_framework versions.

PR #59 had the right idea, but from django.util import six will also not be available anymore

[erseco]

Waiting to this PR to be merged, meanwhile, I added this to my Dockerfile to monkey-patch the library:

# Temporary fix for https://github.com/vbabiy/djangorestframework-camel-case/issues/61
RUN sed -i "s/import six, FormParser/import FormParser/" /usr/lib/python3.6/site-packages/djangorestframework_camel_case/parser.py

[speedy250]

Would love to get this merged and pushed live!!