os.path to pathlib: part 1 / n

[sappelhoff]

• addresses a part for modernize path handling #1268

[sappelhoff]

FYI, I will keep pushing to this for a bit longer, there is still a lot to adjust.

[codecov]

Codecov Report

Attention: Patch coverage is 94.83871% with 8 lines in your changes missing coverage. Please review.

Project coverage is 97.48%. Comparing base (b208a8c) to head (fef7680).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
mne_bids/commands/run.py	0.00%	5 Missing ⚠️
mne_bids/commands/mne_bids_cp.py	33.33%	2 Missing ⚠️
mne_bids/tests/test_copyfiles.py	97.72%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1339   +/-   ##
=======================================
  Coverage   97.47%   97.48%           
=======================================
  Files          40       40           
  Lines        9025     9014   -11     
=======================================
- Hits         8797     8787   -10     
+ Misses        228      227    -1     

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