Allow testing against both Qt5 and Qt6

[aplaice]

In particular we want to:

1. Test both the Qt5 and Qt6 generated python code. (If we're even testing the UI code, which I don't think we are.)
2. Test against Qt5 Anki and Qt6 Anki. (Once we upgrade our Anki dev dependencies, which are sort-of used in testing.)

I think that we can achieve both (in our current mamba tests) by shadowing the Qt6 module (a bit like we currently mock Anki modules). There might also be more elegant solutions.

However, I believe it's low priority.

See: #173.