chore: publish sdist & pure-Python wheel without waiting for binaries

[vytas7]

For 4.0, the setup was the opposite, and that was deliberate -- we waited for all wheels in order to publish anything to PyPI.

However, now that we have a CI gate testing a representative selection of wheels, we should afford to publish sdist first, and then hope that the binary builds succeed too.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (6d7a45b) to head (f759064).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2391   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           64        64           
  Lines         7726      7726           
  Branches      1071      1071           
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  Hits          7726      7726           

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