Add PyQt6 support

[eyllanesc]

Changes:

• [x] PyQt6 is now stricter with enums so QSocketNotifier.Read and QSocketNotifier.Write must be QSocketNotifier.Type.Read and QSocketNotifier.Type.Write, respectively. That does not cause problems with the other bindings.
• [x] Also the exec_ method has been changed to exec.

I have also added a GitHub workflow so you can add the badge to your README: [![test](https://github.com/CabbageDevelopment/qasync/actions/workflows/test.yml/badge.svg)](https://github.com/CabbageDevelopment/qasync/actions/workflows/test.yml)

[TheGreatCabbage]

Awesome, nice work! Great to have the GitHub workflow as well 👍