skpkg: apply black to all files in the project directory

[john-halloran]

This is a stage in the re-cut process for diffpy.snmf. Black was already in use, but did not match the current scikit-package line length.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (john-development@9b0c509). Learn more about missing BASE report.

Additional details and impacted files

@@                 Coverage Diff                 @@
##             john-development     #176   +/-   ##
===================================================
  Coverage                    ?   77.50%           
===================================================
  Files                       ?        3           
  Lines                       ?       40           
  Branches                    ?        0           
===================================================
  Hits                        ?       31           
  Misses                      ?        9           
  Partials                    ?        0           

Files with missing lines	Coverage Δ
tests/test_snmf_optimizer.py	100.00% <100.00%> (ø)
tests/test_version.py	100.00% <ø> (ø)

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