Fix release workflow

[klane]

• Checkout repo in PyPI job
• Only create GitHub releases on tags

[codecov]

Codecov Report

Merging #120 (a402703) into main (39d1360) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #120   +/-   ##
=======================================
  Coverage   18.48%   18.48%           
=======================================
  Files           7        7           
  Lines         395      395           
=======================================
  Hits           73       73           
  Misses        322      322           

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