chore(packaging): set license classifier

[mkniewallner]

Resolves #623.

PR Checklist

• A description of the changes is added to the description of this PR.
• If there is a related issue, make sure it is linked to this PR.
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

Description of changes

We've lost license classifier when switching from Poetry to PDM. Poetry sets that automatically, but PDM does not.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.3%. Comparing base (c9ee643) to head (68114b2).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #624   +/-   ##
=====================================
  Coverage   90.3%   90.3%           
=====================================
  Files         33      33           
  Lines        964     964           
  Branches     188     188           
=====================================
  Hits         871     871           
  Misses        79      79           
  Partials      14      14           

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