[MRG] explicitly declare pymatreader an optional dep for EEGLAB

[sappelhoff]

Follow up to #951 - I think it's better to declare pymatreader an optional dependency, which it will become as soon as we drop mne < 1.0

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>"

[codecov]

Codecov Report

Merging #952 (9d81e37) into main (92258b2) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #952      +/-   ##
==========================================
+ Coverage   94.39%   94.40%   +0.01%     
==========================================
  Files          25       25              
  Lines        3550     3559       +9     
==========================================
+ Hits         3351     3360       +9     
  Misses        199      199              

Impacted Files	Coverage Δ
mne_bids/path.py	97.67% <0.00%> (+0.02%)	⬆️

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[hoechenberger]

Thanks @sappelhoff