Use Black to format Python code

[callahad]

My verification of #6440, following @peterbe's instructions

I ran poetry run black kuma docs tests, as per our .travis.yml file.

I then checked with poetry run flake8 kuma docs tests and saw a lot of errors. Black recommends ignoring the following:

• E203 (whitespace before ':')
• E501 (line length)
• W503 (line break before binary operator)

I manually resolved or added local exceptions for:

• E231 (missing whitespace after ',')
• E731 (do not assign a lambda expression)
• F401 (name imported by unused)
• F405 (name may be undefined or defined from star imports)
• I202 (additional newline in a group of imports)

E231 is due to a bug in Black when collapsing multi-line dicts: psf/black#1202

[codecov-io]

Codecov Report

Merging #6447 into master will increase coverage by 4.33%.
The diff coverage is 93.8%.

@@            Coverage Diff             @@
##           master    #6447      +/-   ##
==========================================
+ Coverage   48.07%   52.41%   +4.33%     
==========================================
  Files         300      300              
  Lines       33827    32898     -929     
  Branches     1764     1860      +96     
==========================================
+ Hits        16264    17242     +978     
+ Misses      17017    15099    -1918     
- Partials      546      557      +11

Impacted Files	Coverage Δ
kuma/users/forms.py	22.36% <ø> (-1.39%)	⬇️
kuma/settings/local.py	18.51% <ø> (+1.85%)	⬆️
kuma/wiki/constants.py	3.11% <ø> (-1.39%)	⬇️
kuma/scrape/tests/test_source_document_redirect.py	65.85% <ø> (+4.49%)	⬆️
kuma/users/tasks.py	53.33% <ø> (+4.35%)	⬆️
kuma/wiki/exceptions.py	88.88% <ø> (ø)	⬆️
kuma/search/views.py	80% <ø> (+6.31%)	⬆️
kuma/core/tests/taggit_extras/models.py	100% <ø> (ø)	⬆️
kuma/dashboards/urls.py	54.54% <ø> (+34.54%)	⬆️
kuma/search/tests/test_tasks.py	100% <ø> (ø)	⬆️
... and 467 more

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[peterbe]

I like this better because it's explicit about the lambda overrides within the code rather than a global exception in the .flake8.