ENH: Add support for PySide6 6.9+

[larsoner]

Following #24825 (comment) I:

• Moved Q* subclasses to last position
• Changed a bunch of if not PYSIDE2 to if not (PYSIDE2 or PYSIDE6) conditionals
• Fixed some incomplete manual __init__ calls for those ☝️

This at least gets me to a window that opens and interacts:

The TODO I think would be:

• Check that all if not (PYSIDE2 or PYSIDE6): conditionals call all super class inits
• Check all Q* classes are at the end (I did things like git grep "QObject, " but this is incomplete)
• Debug and fix the segfault when exiting (not sure how to do this)
• Fix removed sig.emit calls (they cause a segfault)
• Add to CIs

cc @rear1019 in case you want to test and/or contribute. I'm also happy if @ccordoba12 or someone else wants to take over

Closes #20201

[larsoner]

I also see with a slightly better (but still incomplete!) regex:

$ git grep -P "class .*\(Q[A-Z][a-zA-Z]+,.*\)" | wc -l
32

so there are some other candidates to move Q* to the end of class definitions. Not sure if it's needed or helpful, though...