[TESTS] check black style in github actions

[kaczmarj]

Fail the pull request CI if files would be changed when using black.

[codecov]

Codecov Report

Base: 83.67% // Head: 88.77% // Increases project coverage by +5.09% 🎉

Coverage data is based on head (ca428e1) compared to base (95f8f91).
Patch coverage: 84.21% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #501      +/-   ##
==========================================
+ Coverage   83.67%   88.77%   +5.09%     
==========================================
  Files          11       11              
  Lines        1017     1033      +16     
==========================================
+ Hits          851      917      +66     
+ Misses        166      116      -50     

Impacted Files	Coverage Δ
neurodocker/cli/generate.py	91.66% <75.00%> (+0.67%)	⬆️
neurodocker/cli/cli.py	90.00% <100.00%> (-10.00%)	⬇️
neurodocker/cli/minify/trace.py	83.90% <0.00%> (+58.62%)	⬆️

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