Add flake8 hook to pre-commit

[peymanslh]

Add flake8 to pre-commit and remove commented and old configs from the tox file.

[codecov[bot]]

Codecov Report

Merging #537 (edbbf1d) into master (8b9cf49) will not change coverage. The diff coverage is n/a.

:exclamation: Current head edbbf1d differs from pull request most recent head 4e9b7c6. Consider uploading reports for the commit 4e9b7c6 to get more accurate results

@@           Coverage Diff           @@
##           master     #537   +/-   ##
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  Coverage   91.13%   91.13%           
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  Files          28       28           
  Lines        2674     2674           
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  Hits         2437     2437           
  Misses        237      237           

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[hugovk]

We need to update docs/conf.py, something like:

 # The encoding of source files.
-#source_encoding = 'utf-8-sig'
+# source_encoding = 'utf-8-sig'

(Black would have autoformatted these ;)

[peymanslh]

We need to update docs/conf.py, something like:

 # The encoding of source files.
-#source_encoding = 'utf-8-sig'
+# source_encoding = 'utf-8-sig'

Yes, I will fix it. But I excluded docs/ in the flake8 config!(I don't know why it's not working :D)

[hugovk]

I think it's because pre-commit ignores the exclude list and scans all files in Git.

Let's include the docs directory, conf.py is a Python file after all so let's scan it too.