Minor compatibility fixes for PySide6==6.4.2

[jeylau]

Beyond the test for a checkbox state that needed a fix, I couldn't spot anything else; the GUI is functional. It looks to me like we're good to go for an upgrade of PySide6.

[n-poulsen]

Tested, looks good to me. I would just also change the setup.cfg to specify:

[options.extras_require]
testing =
    ...
    pyside6==6.4.2

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (fa2145c) 81.87% compared to head (cfa84b6) 81.87%.

Files	Patch %	Lines
src/napari_deeplabcut/_widgets.py	50.00%	1 Missing ⚠️

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #115   +/-   ##
=======================================
  Coverage   81.87%   81.87%           
=======================================
  Files          11       11           
  Lines        1754     1754           
=======================================
  Hits         1436     1436           
  Misses        318      318           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.