Use `run.patch = subprocess` coverage option

[tjkuson]

coverage 7.10 has a new patch = subprocess option that tracks coverage across subprocesses. pytest-asyncio has some tests that start subprocesses, so if this project were to upgrade and use this new option, I think it'd have more accurate coverage reports.

I ran into this when working on a patch that has code falsely reported as uncovered.

References:

• https://coverage.readthedocs.io/en/latest/config.html#run-patch

[tjkuson]

Actually, I didn't see an improvement locally. Made a mistake, sorry for the noise!