Use SPDX license expression

[EpicWink]

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Only test with Python 3.9+ (due to setuptools support)

[codecov-commenter]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 89.06%. Comparing base (47782f2) to head (d8ab6f7).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #69      +/-   ##
==========================================
- Coverage   89.33%   89.06%   -0.27%     
==========================================
  Files           3        3              
  Lines         741      741              
==========================================
- Hits          662      660       -2     
- Misses         79       81       +2     

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