Black and Python version suport

[bryant-finney]

Summary

Piggy back on #36 for supporting additional black versions (after fixing syntax errors).

Additionally, add support for Python 3.9:

• change requirement on pytest-mypy from ~=0.3 to >=0.3
• add py39 to tox.ini
• synchronize with setup.py

Note that I haven't tested the impacts of removing black from the dependencies.

[codecov-commenter]

Codecov Report

Merging #61 (53bdcc2) into master (2b66c23) will decrease coverage by 0.24%.
The diff coverage is 81.81%.

❗ Current head 53bdcc2 differs from pull request most recent head 02b40de. Consider uploading reports for the commit 02b40de to get more accurate results

@@            Coverage Diff             @@
##           master      #61      +/-   ##
==========================================
- Coverage   94.63%   94.38%   -0.25%     
==========================================
  Files          10       10              
  Lines         745      748       +3     
==========================================
+ Hits          705      706       +1     
- Misses         40       42       +2     

Impacted Files	Coverage Δ
pipenv_setup/setup_updater.py	96.12% <81.81%> (-1.50%)	⬇️

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[jbraswell]

Would love to see this merged instead of my PR at #63.

[bryant-finney]

Oh hey I forgot about this PR 😅 I'll try to push this through later today!