BUG: Fix warning stacklevel

[larsoner]

Similar to #12014, improves this warning:

  /home/larsoner/python/virtualenvs/base/lib/python3.12/site-packages/_pytest/fixtures.py:1303: PytestRemovedIn9Warning: Marks applied to fixtures have no effect

to become:

  /home/larsoner/python/mne-nirs/mne_nirs/conftest.py:133: PytestRemovedIn9Warning: Marks applied to fixtures have no effect

[larsoner]

py313 failures look unrelated

[bluetech]

Hmm if I'm not mistaken (didn't check), the exact depth depends on how many decorators there are? So it's at least 3, since there needs to be at least one mark decorator, but it can be more (such as the test where it's 4).

[larsoner]

I pushed some commits to do a bit of inspect magic to figure out where to emit the warning by traversing the stack until out of the _pytest module. FWIW I adapted it from some code we've used for at least 5 years over in MNE-Python without any real issues. Happy to continue down this path if it seems useful or fine with closing if it seems too magical!

[larsoner]

Yep, too magical -- tests fail because it ends up in the _hypothesis_pytestplugin.py namespace. I'll close!