Limit Tox action on pushes to main

[tsalo]

Closes none.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.19%. Comparing base (50e1c2e) to head (6eb58e6).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #45   +/-   ##
=======================================
  Coverage   61.19%   61.19%           
=======================================
  Files          16       16           
  Lines         951      951           
  Branches      137      137           
=======================================
  Hits          582      582           
  Misses        332      332           
  Partials       37       37           

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