pyside2 is deprecated, use pyside6

[jjakob]

Gentoo has deprecated pyside2: https://packages.gentoo.org/packages/dev-python/pyside2

pyside6 is currently maintained, but requires Qt 6. Maybe there's a way to support both, depending on whether the OS has Qt 5 or 6 available?
https://doc.qt.io/qtforpython-6/gettingstarted/porting_from2.html

[achow101]

IIRC there were/are some issues with building the release binaries since we try to target the oldest systems possible. See #691

[Sjors]

I bumped PyQt5 to PyQt6 as part of #795, if that helps.