Add GitHub action for codecov.

[jsoules]

Note that I've had to use the Python codecov app (as done in bayes-kit) instead of using the codecov workflow, which appears pretty flaky. (The solution mentioned there is to add a secret token, which I've already done; and we can already demonstrate that it isn't a rate-limiting issue since an earlier step can find the files which we're trying to upload.)

In any event, this should now be working, though I may have to tweak to get it to comment coverage onto PRs. (This PR is partially testing that.)

[codecov-commenter]

Codecov Report

:exclamation: No coverage uploaded for pull request base (main@cd17ff9). Click here to learn what that means. The diff coverage is n/a.

@@          Coverage Diff           @@
##             main     #51   +/-   ##
======================================
  Coverage        ?   5.63%           
======================================
  Files           ?      60           
  Lines           ?    5359           
  Branches        ?      21           
======================================
  Hits            ?     302           
  Misses          ?    5057           
  Partials        ?       0           

Flag	Coverage Δ
gui_units	6.81% <0.00%> (?)
svc_units	2.34% <0.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

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[jsoules]

Looks like that worked (although obviously there's no prior coverage report from which to do a diff). May need to tweak the config a bit since it may be excluding from the denominator all files with no tests whatsoever...

[jsoules]

This should now be ready for merging upon review.

[magland]

Looks fine to me.