chore: rework single python tests

[mayeut]

Seeing the many tests that were changed in #1456 to move from one python version to another, we might want a helper for that so that we can do the change in a single location.

Using a single python version is extended to the following tests in order to reduce CI load:

• test_before_all.py
• test_before_build.py:test_cwd
• test_build_skip.py (by updating the skip pattern for the build)
• test_cpp_standards.py (even though "python version independent" wasn't quite true for PyPy at some point)
• test_environment.py
• test_musllinux_X_Y_only.py
• test_testing.py: test_extras_require

There are also 1 other commits to reduce the load on CI for test_abi3.

[mayeut]

I'll apply the same review suggestion as in #1834 (I re-used the same pattern once more in test/test_before_all.py)