Add pypy support

[vivekjoshy]

Affirmation

By submitting this Pull Request or typing my (user)name below,
I affirm the Developer Certificate of Origin
with respect to all commits and content included in this PR,
and understand I am releasing the same under openskill.py's MIT license.

I certify the above statement is true and correct: @daegontaven

[codecov]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage 👍

Coverage data is based on head (e970cb6) compared to base (a1acf12).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #80   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           11        11           
  Lines          605       622   +17     
  Branches       145       149    +4     
=========================================
+ Hits           605       622   +17     

Impacted Files	Coverage Δ
openskill/rate.py	100.00% <100.00%> (ø)
openskill/statistics.py	100.00% <100.00%> (ø)
openskill/util.py	100.00% <100.00%> (ø)

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