replace fill: parameter with logic to expand py3* glob syntax in envs

[zacharyburnett]

as this change removes a parameter it will require a major version bump

better solution for #325 than #326

replaces the fill: parameter with just specifying py3* or py31* in the env name to expand into supported Python versions; I should have done this originally in #326 😅

this change also moves supported_pythons.py into tox_matrix.py

[zacharyburnett]

the automated tests failed yesterday due to YET ANOTHER github outage 🙄 I'll try to rerun them

[zacharyburnett]

standby, I need to clean this up a bit and then it'll be ready

[zacharyburnett]

there, done

[zacharyburnett]

this moves the "retrieve latest Python versions list from the Internet" functionality from supported_pythons.py to the zacharyburnett/supported-pythons action. I've published this version of supported-pythons to the github actions marketplace, so it should be unaffected if I leave STScI or delete my account or whatever; however, we can also move the entire action repo to OpenAstronomy/supported-pythons if that's preferable

[Cadair]

I merged #397 and now it needs a rebase sorry 🙈

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.66%. Comparing base (e113052) to head (65a67fc).
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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #392   +/-   ##
=======================================
  Coverage   66.66%   66.66%           
=======================================
  Files           2        2           
  Lines           6        6           
=======================================
  Hits            4        4           
  Misses          2        2           

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