Support PyQt6 (default in Qubes 4.3)

[deeplow]

Description

PyQt6 is available by default in Qubes 4.3. Theoretically we could also install PyQt5 there, but if we can spare dom0 from another dependency, that's a positive. Therefore we should make our current code work with both versions, at least while Qubes 4.2 is around.

Relevant PyQt5 → PyQt6 changes

They are practically all backwards compatible, but some are not forward compatible. In order words, PyQt6 is more stringent, but this should keep working on PyQt5. Here are some of the relevant changes to be adopted:

• replace "exec_()" with exec() (Qt5 targeted also python2, which meant that it had "exec" reserved. This is no longer the case, so we can use the more aptly named "exec()" (NOTE: check if this works on PyQt5, otherwise add a try...except)
• Expand enums like QMessageBox.Ok to have its full name: QMessageBox.StandardButtons.Ok

A good overview of the upgrade notes can be found at https://www.pythonguis.com/faq/pyqt5-vs-pyqt6/

[deeplow]

Just for context, I did some digging and there was a similar issue that happened in the past where both PyQt4 and PyQt5 had to be supported.

In particular, I'm thinking of using try...except ImportError, but in the past this was done via an environment variable: SDW_UPDATER_QT