Add support for qt6 (PyQt6 and PySide6)

[ldet]

Hello,
i added PySide6 and PyQt6 support to the -qt-support command line argument. The changes from qt5 to qt6 where quite minimal so I mostly added imports. The only relevant breaking change is the deprecation (PySide6) / removal (PyQt6) of exec_() in favor of exec() in QThread/QCoreApplication.
PyQt5 and PySide2 support both, so once Qt4 support is dropped, that additional check could be removed.

test_case_qthread4 is failing for me with PySide2 and PySide6, so i dont think its actually related to my change. It works with PyQt6 though (only tested with those three).

I did not touch any of that pydev_ipython stuff, because i never really used IPython, but I had a quick look at the upstream sources in the IPython repo and they changed some other things, so it might be worth importing those again.