Infrastructure clean-up

[pllim]

Note to whoever merges this: Please use Squash and Merge button.

Stuff needs modernization.

• Where is codecov bot?
• Switch to OpenAstronomy
• Add publish workflow and have it also run in PR

Need to do this after merge:

• Add PYPI_TOKEN to repo secrets #607

Would make the following fixes easier going forward:

• Use dateutil.tz over pytz ? #601
• Replace deprecated plot_date #603

Might naturally close the following when you next do a release after merging this:

• WebAssembly ImportError - add whl to PyPI #583

[codecov]

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