Clarify that perfect groups data is licensed under GPL 2.0 or later

[hulpke]

Wrote more general licensing text to appease GPL sherrifs.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.88%. Comparing base (228e23a) to head (28fc315).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6450      +/-   ##
==========================================
- Coverage   78.89%   78.88%   -0.01%     
==========================================
  Files         685      685              
  Lines      293519   293518       -1     
  Branches     8667     8667              
==========================================
- Hits       231561   231550      -11     
  Misses      60162    60162              
- Partials     1796     1806      +10     

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