This is just a minimally reproducible example of using pytest-qt to test a simple widget. These are the two ways I've tried running the tests.

pytest directly

$ python -m venv .venv
$ source .venv/bin/activate.fish
(.venv)$ pip install pyqt5 pytest pytest-qt
(.venv)$ python setup.py develop
(.venv)$ python -m pytest

This results in a successful test run.

tox

$ tox

This results in an InvocationError with no further explanation.