chore(core): Introduce black code formatter

[ceache]

Use black to format the code

Why is this needed?

Remove friction during development caused by styling differences.
Minimize diffs.

Proposed Changes

• Format all code with black
• Add a style check to the gh action

Does this PR introduce any breaking change?

No

[codecov-commenter]

Codecov Report

Base: 94.39% // Head: 94.40% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (644eeb7) compared to base (cb2fe85).
Patch coverage: 89.36% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #673      +/-   ##
==========================================
+ Coverage   94.39%   94.40%   +0.01%     
==========================================
  Files          57       57              
  Lines        8399     8399              
==========================================
+ Hits         7928     7929       +1     
+ Misses        471      470       -1     

Impacted Files	Coverage Δ
kazoo/python2atexit.py	40.62% <0.00%> (ø)
kazoo/recipe/election.py	100.00% <ø> (ø)
kazoo/tests/conftest.py	42.85% <0.00%> (ø)
kazoo/tests/test_eventlet_handler.py	88.52% <0.00%> (-1.64%)	⬇️
kazoo/tests/test_gevent_handler.py	80.28% <0.00%> (ø)
kazoo/tests/test_cache.py	57.48% <26.38%> (ø)
kazoo/tests/test_election.py	91.30% <50.00%> (ø)
kazoo/tests/test_build.py	87.50% <66.66%> (ø)
kazoo/testing/common.py	78.78% <72.41%> (ø)
kazoo/tests/util.py	62.50% <75.00%> (-1.39%)	⬇️
... and 57 more

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[ceache]

@StephenSorriaux rebased and ready for review

[StephenSorriaux]

Thank you!