Run tests on wheels

[reinecke]

This change re-shuffles the CI so that:

• A "smoke test" set of builds are run on one OS and generate coverage
• A separate "build and test only" set of platforms/python version run off the sdist (currently mingw)
• All wheels are built after smoke tests pass
• The test suite is run for each OS/python combo against the wheels

Ideally this would take the suggestion from @JeanChristopheMorinPerso and run against sdist - that change hasn't been made.
Update: after investigation, cibuildwheel isn't really meant to happen from sdist sources. I did a pass to make our build and test only phase run off it though - since those platforms would be running off sdist in the wild.

Additionally, if we could capture coverage from one of the cibuildwheel runs, we could probably eliminate the smoke test phase.

• Consider dropping the smoke test and just harvest coverage from testing the wheels
  • Update: we decided against this because you need particular build options to generate needed coverage artifacts
• Investigate having the wheel build to happen off the sdist
• Currently the build and test only phase is hitting a test error - getting it to find the sdist tar.gz when the name is indeterminate (includes the version) is proving tricky
• Re-shuffle the order of repo checkout and wheel install in the wheel test action to match the test only step
• Reduce the matrix on the smoketest build so that it only does the one it needs for coverage - consider renaming explicitly as "generate_coverage" or similar

Fixes #1027
This supersedes #1961 - the changes have been added to this PR

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.15%. Comparing base (ad449d8) to head (9ef3995).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1977      +/-   ##
==========================================
+ Coverage   85.13%   85.15%   +0.01%     
==========================================
  Files         181      181              
  Lines       12783    12783              
  Branches     1206     1206              
==========================================
+ Hits        10883    10885       +2     
+ Misses       1717     1715       -2     
  Partials      183      183              

Flag	Coverage Δ
py-unittests	85.15% <ø> (+0.01%)	⬆️

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see 1 file with indirect coverage changes

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[reinecke]

I'm ready for feedback on this - just the one open questions about whether or not we really need the smoketests or if I should move the coverage reports to the tests we run on the equivalent wheel.