Add SPDX classifier for the license

[sk1p]

According to https://packaging.python.org/en/latest/guides/writing-pyproject-toml/ nowadays you should have license and license-files metadata entries these days.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 51.51%. Comparing base (de7caca) to head (d584a31).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #76   +/-   ##
=======================================
  Coverage   51.51%   51.51%           
=======================================
  Files           1        1           
  Lines         594      594           
  Branches      112      112           
=======================================
  Hits          306      306           
  Misses        274      274           
  Partials       14       14           

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[uellue]

Thank you for taking care of this, LGTM! :-)