MAINT: Migrate from PyQt5 to PySide6

[florin-pop]

Fixes https://github.com/mne-tools/mne-nirs/issues/471 by:

• replacing PyQt5 with pyside6 in requirements.txtwhich is also used by Circle CI / Azure pipelines
• replacing PyQt6 with pyside6 for github workflows
• adding a mitigation for ensuring that some tests are not failing due to a QT deprecation warning

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Contributed with ❤️ by AE Studio

[codecov[bot]]

Codecov Report

Merging #510 (13d4819) into main (2f7ceb8) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #510      +/-   ##
==========================================
+ Coverage   95.36%   95.37%   +0.01%     
==========================================
  Files          69       69              
  Lines        2781     2791      +10     
  Branches      397      400       +3     
==========================================
+ Hits         2652     2662      +10     
  Misses         65       65              
  Partials       64       64              

Impacted Files	Coverage Δ
mne_nirs/tests/test_examples.py	81.25% <100.00%> (+1.25%)	:arrow_up:
mne_nirs/visualisation/tests/test_visualisation.py	100.00% <100.00%> (ø)