cpp wrapper generator fails on Qt 5.15.2

[davidkhess]

The generation process fails to properly parse and find all of the necessary classes. See here for more details:

https://github.com/MeVisLab/pythonqt/issues/13#issuecomment-752737191

@florianlink just opening this as a standalone issue so it doesn't fall through the cracks. I understand it's not necessarily a priority at the moment.