build: drop support for python 3.8, test 3.12

[tlambert03]

this also pins pyside6 to 6.7.3 ... still quite hard to find a good match there

this also stops testing the experimental stack viewer on PySide2/6, since those tests fail too often, and we'll likely move to ndv

[codecov]

Codecov Report

Attention: Patch coverage is 95.83333% with 1 line in your changes missing coverage. Please review.

Project coverage is 90.71%. Comparing base (a585acb) to head (ea5130d).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...mcore_widgets/views/_stack_viewer/_stack_viewer.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #368      +/-   ##
==========================================
- Coverage   90.72%   90.71%   -0.01%     
==========================================
  Files          85       85              
  Lines        9185     9188       +3     
==========================================
+ Hits         8333     8335       +2     
- Misses        852      853       +1     

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