Use correct URL for test-pypi for publishing test wheels

[eriknw]

Continuation of #607

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (a39cd67) to head (da989be).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #608   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           13        13           
  Lines         1181      1181           
=========================================
  Hits          1181      1181           

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