Combine Windows and Pytest workflows using YAML anchors

[michaelosthege]

There is considerable duplication of YAML code between the pytest.yml and windows.yml workflows.

With #4512 the splitting of tests across jobs is cleaned up. Exactly this new matrix definition should be refactored into a YAML anchor such that the Windows job from windows.yml can be migrated into pytest.yml and also run all the tests.

I would add the "beginner friendly" label, but I find YAML so unbelievably hard to work with that it wouldn't be fair.