MRG: Rework CI test matrix

[hoechenberger]

Fixes #933

Merge checklist

Maintainer, please confirm the following before merging:

• All comments resolved
• This is not your own PR
• All CIs are happy
• PR title starts with [MRG]
• whats_new.rst is updated
• PR description includes phrase "closes <#issue-number>"

[hoechenberger]

Ok, so this reduces the number of GH Actions tests from 20 to 12. 👍

[codecov]

Codecov Report

Merging #934 (635a610) into main (8a29504) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #934   +/-   ##
=======================================
  Coverage   94.78%   94.78%           
=======================================
  Files          23       23           
  Lines        3488     3490    +2     
=======================================
+ Hits         3306     3308    +2     
  Misses        182      182           

Impacted Files	Coverage Δ
mne_bids/utils.py	95.91% <0.00%> (-0.52%)	⬇️
mne_bids/read.py	95.82% <0.00%> (+0.01%)	⬆️
mne_bids/path.py	97.64% <0.00%> (+0.14%)	⬆️

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[sappelhoff]

nice! Thanks :-)

[sappelhoff]

thanks richard