Do not pin black to avoid conflict with consumers

[jbraswell]

This was forcing us to use an older version of black. Also, because black is still in pre-release, had to add allow_prereleases to Pipfile.

Alternatively, since the code is written to fallback to autopep8 if import black fails, we could just remove the dependency on black entirely.

[codecov-commenter]

Codecov Report

Merging #63 (42d7972) into master (2b66c23) will decrease coverage by 0.67%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #63      +/-   ##
==========================================
- Coverage   94.63%   93.95%   -0.68%     
==========================================
  Files          10       10              
  Lines         745      745              
==========================================
- Hits          705      700       -5     
- Misses         40       45       +5     

Impacted Files	Coverage Δ
pipenv_setup/setup_updater.py	93.65% <0.00%> (-3.97%)	⬇️

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[jbraswell]

Not sure codecov/project is claiming reduced coverage for pipenv_setup/setup_updater.py.

[bryant-finney]

This PR looks related to #75