Add license attribute to gemspec

[bastelfreak]

No description provided.

[codecov-commenter]

Codecov Report

Merging #44 (ff39f3e) into main (276aa0a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #44   +/-   ##
=======================================
  Coverage   95.40%   95.40%           
=======================================
  Files           2        2           
  Lines         174      174           
=======================================
  Hits          166      166           
  Misses          8        8           

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