Currently it's not possible to run tox locally on an arm64 machine (e.g. a moder MacBook pro). This is because all deps are installed from PyPI, and there are no PyQT5 wheels available for arm64 there.

Two options I can see here:

• See if it's possible to use PySide of PyQT5 instead for testing
• Install PyQT5 from conda using testing using tox-conda.

I'll investigate both of these and try and get a fix working.