QEvent.Type error seen on Windows 10 + Python 3.8 + PyQt6 backend

While testing the latest `trauitsUI` on Python 3.8 using the `PyQt6` backend (all provided via EDM), I see the following traceback (truncated to just the part with the relevant line):

```
...

_QT_TRAITS_EVENT = QtCore.QEvent.Type(QtCore.QEvent.registerEventType())
File "C:\Users\\.edm\envs\\lib\enum.py", line 339, in __call__
return cls.__new__(cls, value)
File "C:\Users\\.edm\envs\\lib\enum.py", line 663, in __new__
raise ve_exc
ValueError: 65534 is not a valid Type
```

The above stems from this line in `traitsui`: https://github.com/enthought/traitsui/blob/main/traitsui/qt4/toolkit.py#L49