Allow overriding Python version matrix

[teutoburg]

Useful when updating supported Python versions for different packages in the ecosystem one at a time...

Annoyingly, the defaults now have to be stored in two places, because of different contexts when running on push/PR here vs. when called from elsewhere. Alternative solution would be to use these strictly as called workflows, and have another workflow that acts as the caller on push/PR, much like these are called from elsewhere anyway. I think that's also how we solved a similar issue in the IRDB. But we can always come back and do that, it wouldn't change the outside call signature...

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (c10096e) to head (9b42a27).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #88   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines            7         7           
=========================================
  Hits             7         7           

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[teutoburg]

Unfortunately I kind of urgently need this in Scopesim-Targets, because I need to drop Python 3.10 there. So I'll just merge this and deal with potential issues afterwards. Sorry.