Blackify code, Hypothesis testing

[zhudotexe]

Summary

This adds property-based testing using Hypothesis in order to test arbitrary dice strings.

It also blackifies the code and updates the CI to test on the latest versions of Python.

Note: Hypothesis testing takes a while, so builds will probably take much longer

Checklist

PR Type

• [ ] This PR is a code change that implements a feature request.
• [ ] This PR fixes an issue.
• [ ] This PR adds a new feature that is not an open feature request.
• [x] This PR is not a code change (e.g. documentation, README, ...) - build and formatting only

  Other

• [ ] This PR is a breaking change (e.g. methods or parameters removed/renamed)
• [x] If code changes were made then they have been tested.
• [ ] I have updated the documentation to reflect the changes.

[codecov[bot]]

Codecov Report

Merging #9 (9315a61) into master (591b1aa) will increase coverage by 0.33%. The diff coverage is 97.43%.

@@            Coverage Diff             @@
##           master       #9      +/-   ##
==========================================
+ Coverage   94.52%   94.86%   +0.33%     
==========================================
  Files           8        8              
  Lines         895      895              
==========================================
+ Hits          846      849       +3     
+ Misses         49       46       -3     

Impacted Files	Coverage Δ
d20/expression.py	96.05% <85.71%> (+0.65%)	:arrow_up:
d20/dice.py	94.36% <100.00%> (ø)
d20/diceast.py	91.63% <100.00%> (ø)
d20/errors.py	100.00% <100.00%> (ø)
d20/stringifiers.py	100.00% <100.00%> (+1.23%)	:arrow_up:
d20/utils.py	98.00% <100.00%> (ø)

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