CI: Use PyPA/build to set up source distribution in deploy

[bocklund]

• Use PyPA/build to build distributions as recommended by setuptools. The source distribution builds and passes twine check as verified here.
• Update pyproject.toml to explicitly use the legacy setuptools backend, which is required since we vendor a local versioneer and cannot do full PEP 517 build isolation.
• Add long_description_content_type to our setup.py to pass twine check

[codecov]

Codecov Report

Merging #330 (d9112b7) into develop (db5086c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #330   +/-   ##
========================================
  Coverage    87.34%   87.34%           
========================================
  Files           46       46           
  Lines         4521     4521           
========================================
  Hits          3949     3949           
  Misses         572      572           

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[richardotis]

Thanks for working this. Does this ensure the tests are run and pass before a tagged commit is built and uploaded?

[bocklund]

This particular workflow does not do that. I don't really see it as blocking because:

1. The current releasing workflow where developers upload source distributions manually doesn't necessarily ensure that the tests pass in CI
2. You can mitigate and ensure the tests pass on CI "manually" by pushing the commit you're planning to tag first to see if the tests pass. If they pass, you can tag and git push origin master --tags and trigger this workflow.

[richardotis]

I agree that it's not a regression to do it this way and that it's fine to move forward. I would eventually like to test the exact artifacts we're uploading to PyPI, particularly once we start building platform wheels.