add deprecation warning for specifying strict_exception_groups=False

[jakkdl]

Wasn't totally sure which issue to refer to, ended up going with #2929

the removed with block in test_subprocess was added to try and track down some weirdness in #2886 (comment) - but I don't think it served much of a purpose in the end, so opted now to remove it instead of wrapping it in a pytest.warns.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b6a31d7) 99.64% compared to head (06168d2) 99.64%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2941   +/-   ##
=======================================
  Coverage   99.64%   99.64%           
=======================================
  Files         116      117    +1     
  Lines       17521    17564   +43     
  Branches     3151     3166   +15     
=======================================
+ Hits        17459    17502   +43     
  Misses         43       43           
  Partials       19       19           

Files	Coverage Δ
src/trio/_core/_run.py	99.66% <100.00%> (+<0.01%)	⬆️
src/trio/_core/_tests/test_run.py	100.00% <100.00%> (ø)
...ts/test_deprecate_strict_exception_groups_false.py	100.00% <100.00%> (ø)
src/trio/_tests/test_subprocess.py	99.23% <ø> (-0.01%)	⬇️

[Zac-HD]

Looks good - thanks @jakkdl!

[CoolCat467]

Making sure, nothing can go wrong with the literal is False compare? No one could set strict exception groups to a value that could be interpreted as the False branch?

[jakkdl]

Making sure, nothing can go wrong with the literal is False compare? No one could set strict exception groups to a value that could be interpreted as the False branch?

Right, if somebody is supplying a falsy value of an incorrect type they're not gonna get a warning. So I suppose I could do

if not strict_exception_groups and strict_exception_groups is not None:
    ...

Don't think we need to be that defensive, but I guess it also doesn't hurt.