Add a PySide6 backend/example

[abey79]

PySide2 appears to be no longer maintained and PySide6 is obviously the future-proof Python/Qt binding. More specifically for mac users, as of PySide6 6.2.2 released yesterday (Dec 3rd), it's the only Qt binding offering a working, arm64-native binary distribution on PyPI.

Unfortunately, GL integration is one of the few area where great changes were made between Qt5 and Qt6. Specifically, QGLWidget had been replaced by QOpenGLWidget, which integrates differently with external rendering libraries.