Add types to trio._subprocess

[TeamSpen210]

Another one of the remaining modules. During typing I did find two type issues:

• The IO modules might need slightly looser types - wait_readable() and similar method should take int | HasFileno, not just int | socket.
• subprocess.Popen takes str | bytes | os.PathLike or a Sequence of those, but open_process() does an isinstance(..., str) check so it won't correctly accept those.

[codecov]

Codecov Report

Merging #2753 (614297f) into master (dcec304) will increase coverage by 0.00%.
Report is 1 commits behind head on master.
The diff coverage is 98.24%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2753   +/-   ##
=======================================
  Coverage   98.92%   98.92%           
=======================================
  Files         113      113           
  Lines       16717    16727   +10     
  Branches     3026     3026           
=======================================
+ Hits        16537    16547   +10     
  Misses        124      124           
  Partials       56       56           

Files Changed	Coverage Δ
trio/_subprocess_platform/waitid.py	70.73% <85.71%> (+1.50%)	⬆️
trio/_subprocess.py	97.03% <100.00%> (+0.09%)	⬆️
trio/_subprocess_platform/kqueue.py	100.00% <100.00%> (ø)
trio/_subprocess_platform/windows.py	100.00% <100.00%> (ø)

[TeamSpen210]

Missing coverage is an existing issue, trio._subprocess_platform.waitid.wait_child_exiting() doesn't seem to be exercised by any tests.

[jakkdl]

The IO modules might need slightly looser types - wait_readable() and similar method should take int | HasFileno, not just int | socket.

go ahead and change those if you want, I think int|socket was just a qualified guess of mine and only looking at usage within the same file

subprocess.Popen takes str | bytes | os.PathLike or a Sequence of those, but open_process() does an isinstance(..., str) check so it won't correctly accept those.

I don't see any reason not to make the isinstance check wider?

[jakkdl]

mostly just a bunch of smallish nits