Support PyQt6

1. Improve the check for existing Qt frameworks by importing QtCore from them. This is useful because sometimes pip uninstall pyqt5 can leave cruft behind so that the import succeeds, but importing any real functionality will fail.
2. Add PyQt6 to the list of frameworks to try after PySide6. Same conditionals apply.

Not 100% sure it's correct, I'm building a wheel and I'll try testing it shortly. Let me know if this seems like it's on the right track, though!