Fix test suite failing at collection on Python 3.10 in Pytest 7.x

[CAM-Gerlach]

Possibly due to an updated (packaging or pytest) dependency, it looks like imports of the installed qtpy package started failing to pick up the package in recent QtPy CI test runs on Python 3.10, all platforms and install methods (it seems plausible Python 3.6 isn't failing due to not supporting the problem version of said dependency). I'm pretty sure what to look at here, so I'll go ahead and try to fix it ASAP.

[CAM-Gerlach]

After much testing, I've been able to reproduce the problem locally and isolated the problem to a regression in Pytest 7.0,0, pytest-dev/pytest#9645 with --importmode=importlib. I can confirm that it is fixed by pytest-dev/pytest#9681 , which is merged, but unfortunately they haven't yet cut another release (which will presumably be 7.0.2). For now, we can either not use --importmode=importlib, or perhaps better, just restrict pytest to ==6.* and >7.0.2.