Coverage and diffcov tests are unreliable

[jaraco]

“Collateral” tests fail with:

importlib_resources/functional.py (62.5%): Missing lines 3,5,8,11,16,23,28,35,40,48,63,78
importlib_resources/tests/test_functional.py (98.2%): Missing lines 7,9

These are top-level statements (mostly def), and an alternate import for old Python versions.

Originally posted by @encukou in #303 (comment)

[jaraco]

They're unfortunately inaccurate because pytest (or a plugin or one of the dependencies) imports importlib_resources prior to starting up coverage, so any lines "executed" at coverage time get "missed". I did some more digging and found that pytest enabler unconditionally imports importlib_resources, probably because it takes advantage of features in importlib_resources 5.10, which didn't land in CPython until 3.12. There may be other plugins implicated too.

[jaraco]

I've released a new pytest-enabler 3.1, which relies on the stdlib for importlib_resources in Python 3.12. Now when running the tests, they show a much more accurate picture of the coverage (on Python 3.12+). This issue still affects other dependencies used by pytest-enabler (jaraco.context, jaraco.functools). I've opened pytest-dev/pytest-cov#635 to track a more comprehensive solution, but I'm closing this for now.