Support PySide6 v6.8.1

[ptsavol]

This PR adds support for PySide6 6.8.1.

Fixed crashes by doing the following:

• Inserted homeless Slots into a class that inherits from QObject in spine_engine_worker.py
• Removed connecting the 'finished' by using lambda in project.py

Fixes #2980

Checklist before merging

• Documentation is up-to-date
• Release notes have been updated
• Unit tests have been added/updated accordingly
• Code has been formatted by black & isort
• Unit tests pass

[codecov]

Codecov Report

Attention: Patch coverage is 69.44444% with 22 lines in your changes missing coverage. Please review.

Project coverage is 67.13%. Comparing base (ca3e446) to head (a6831f4).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
spinetoolbox/spine_engine_worker.py	67.74%	17 Missing and 3 partials ⚠️
spinetoolbox/widgets/settings_widget.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3024      +/-   ##
==========================================
- Coverage   67.15%   67.13%   -0.02%     
==========================================
  Files         193      193              
  Lines       37066    37071       +5     
  Branches     4445     4445              
==========================================
- Hits        24890    24888       -2     
- Misses      10919    10926       +7     
  Partials     1257     1257              

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