[MNT] pypi release only on GH release

[fkiraly]

This PR ensures that the release workflow is triggered only on GitHub release.

Currently, it seems the release is triggered on every push to master - though it has thankfully failed for the past merges.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.14%. Comparing base (e2bf3fd) to head (a3ba1dd).
Report is 1 commits behind head on master.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1656   +/-   ##
=======================================
  Coverage   90.14%   90.14%           
=======================================
  Files          32       32           
  Lines        4780     4780           
=======================================
  Hits         4309     4309           
  Misses        471      471           

Flag	Coverage Δ
cpu	90.14% <ø> (ø)
pytest	90.14% <ø> (ø)

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