Update for PyQt6 compatibility

PyQt6 has been released but pytest-qt does not yet support the new version. This causes most tests in my project to fail with the following error, though the application works as expected.

Example error (last few lines only):

INTERNALERROR>   File "/usr/local/var/pyenv/versions/3.9.0/envs/mn390/lib/python3.9/site-packages/pytestqt/qt_compat.py", line 83, in set_qt_api
INTERNALERROR>     raise RuntimeError(msg)
INTERNALERROR> RuntimeError: pytest-qt requires either PySide, PySide2, PyQt4 or PyQt5 to be installed
INTERNALERROR>   PyQt4.QtCore: No module named 'PyQt4'
INTERNALERROR>   PyQt5.QtCore: No module named 'PyQt5'
INTERNALERROR>   PySide.QtCore: No module named 'PySide'
INTERNALERROR>   PySide2.QtCore: No module named 'PySide2'

[The-Compiler]

@othalan @GuillaumeFavelier @rbreu If anyone of you has a PyQt6 project to test with, it’d be great if you could try #340. For my own project I’m still waiting for QtWebEngine to come back with Qt 6.2, but at least pytest-qt’s selftests now work with PyQt6 🎉

[rbreu]

Hi, I just wanted to let you know that I have ported all my unit tests from my previous workaround to pytest-qt and it works. Thank you so much!