MRG: Drop support for MNE-Python <1.3, add new CI job to cover "previous stable" MNE version

[hoechenberger]

PR Description

We commonly only support the latest 2 stable versions of MNE-Python. Hence I'm doing some related cleanups here.

Merge checklist

Maintainer, please confirm the following before merging.
If applicable:

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

[hoechenberger]

@sappelhoff Turns out a number of tests in test_eegieeg were actually broken as they were allowed to xfail too broadly. I tried to make the tests DRY'er and fixed all the problems I found along the way.

[codecov]

Codecov Report

Merging #1140 (f32b746) into main (a3bf0b0) will increase coverage by 2.31%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1140      +/-   ##
==========================================
+ Coverage   95.28%   97.59%   +2.31%     
==========================================
  Files          40       40              
  Lines        8699     8577     -122     
==========================================
+ Hits         8289     8371      +82     
+ Misses        410      206     -204     

Impacted Files	Coverage Δ
mne_bids/commands/tests/test_cli.py	100.00% <100.00%> (+3.38%)	⬆️
mne_bids/tests/test_read.py	99.71% <100.00%> (+1.23%)	⬆️
mne_bids/tests/test_write.py	99.29% <100.00%> (+8.95%)	⬆️

... and 1 file with indirect coverage changes

[hoechenberger]

Ha! Look at that coverage boost!

[sappelhoff]

LGTM! Thanks :)