Enforcing of flake8 linter

[sliverc]

When I created PR #347 I noticed many flake8 linting errors and it was difficult to differentiate between the errors I have possibly made in my new code block and errors which have been there before.

To avoid this and improve code quality I have adjusted the code so flake8 linting is enforced by CI. I have used autopep8 and some manual adjustments to do so.

Following flake8 flags changed:

• Allow F405 (star imports) as this is part of drf json api concept
• Remove E501 so defined max-line-length won't be ignored (otherwise max-line-length doesn't make any sense)

[codecov-io]

Codecov Report

Merging #348 into develop will increase coverage by 0.4%.
The diff coverage is 77.31%.

@@            Coverage Diff             @@
##           develop     #348     +/-   ##
==========================================
+ Coverage    76.45%   76.86%   +0.4%     
==========================================
  Files           50       50             
  Lines         6036     6013     -23     
==========================================
+ Hits          4615     4622      +7     
+ Misses        1421     1391     -30

Impacted Files	Coverage Δ
example/api/serializers/post.py	100% <ø> (ø)	⬆️
example/tests/test_multiple_id_mixin.py	100% <ø> (ø)	⬆️
example/factories/__init__.py	97.67% <ø> (ø)	⬆️
example/tests/test_sideload_resources.py	100% <ø> (ø)	⬆️
example/tests/test_model_viewsets.py	100% <ø> (ø)	⬆️
rest_framework_json_api/pagination.py	54.94% <ø> (-0.2%)	⬇️
example/serializers.py	100% <ø> (ø)	⬆️
example/tests/test_generic_validation.py	100% <ø> (ø)	⬆️
example/tests/test_format_keys.py	100% <ø> (ø)	⬆️
example/tests/test_generic_viewset.py	100% <ø> (ø)	⬆️
... and 36 more

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[mblayman]

@sliverc, This is really good stuff! Thank you very much for this contribution. Could I make two additional requests?

• Could you add your name to the AUTHORS file to acknowledge your contribution? This is a community project and I think everyone should be recognized for their work.
• Could you add an entry to the CHANGELOG for version v2.3.0?

Once those things happen, I'm happy to merge this. 👍

[sliverc]

@mblayman Thanks. Adjusted as requested.