Fix creating new QEvent types in PyQt6

To handle trait change events which are dispatched via the ui, Pyface (and in deprecated code, TraitsUI) define a new event type using the construct:
```
QtCore.QEvent.Type(QtCore.QEvent.registerEventType())
```
This doesn't work on PyQt6. Ideally there is some cross-backend invocation which will work, but at a minimum we need to use an appropriate method for PyQt6 when in that backend. It may be sufficient to check the value of the type, for example.