Migrate to PySide6

[abey79]

Description

Why:

• Much easier installation for Apple silicon Macs (vpype[all] can't be installed on Mac OSX on ARM based Macs (M1) #320)
• Better compatibility with various installs of the interpreter (e.g. Homebrew m1 mac w/ homebrew workaround; display glitches #473)
• Better perspectives of Python 3.11+ compatibility
• General better future-proof-ness

How:

• QGLWidget deprecated and must be replaced by QOpenGLWidget. The main required change was to defer mcl-related initialisation to the first call of paintGL(). Doing it in initializeGL() no longer works.
• The mere importing of PySide6.QtGui now relies on some OpenGL libraries (possibly due to these bugs). This broke the ubuntu GitHub Actions runner and requires explicit installation of mesa stuff (sudo apt-get install -y -qq libegl1-mesa libegl1-mesa-dev).
• The already poor typing stubs of PySide2 somehow got worse, so lots of # typing: ignore to be added.
• Various other easy-to-fix API fixes.
• PySide6 broke mypy due to new enums (see Fix viewer resizing issue #559)
• Further fix in 5a3b403

Also, PySide6 introduces a regression when using window manager such as Rectangle and QOpenGLWidget. The window blanks when resized. See this bug report bug report.

Note: this PR doesn't include updated installation instruction, to be done separately close to release date to minimise confusion (#554)

Checklist

• feature/fix implemented
• code formatting ok (black and isort)
• mypy returns no error
• tests added/updated and pytest succeeds
• documentation added/updated
  • command docstring and option/argument help
  • README.md updated (Feature Overview)
  • CHANGELOG.md updated
  • added new command to reference.rst
  • RTD doc updated and building with no error (make clean && make html in docs/)

[codecov]

Codecov Report

Base: 94.72% // Head: 94.72% // No change to project coverage 👍

Coverage data is based on head (a7c8371) compared to base (91d32f9).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #552   +/-   ##
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  Coverage   94.72%   94.72%           
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  Files          62       62           
  Lines        5668     5668           
  Branches      765      765           
=======================================
  Hits         5369     5369           
  Misses        182      182           
  Partials      117      117           

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[sonarqubecloud]

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0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication