Allow tox to be run locally on arm64 macOS

[dstansby]

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.

[dstansby]

Looks like another backend option is PyQT6, which has macOS universal wheels: https://pypi.org/project/PyQt6/#files

[dstansby]

Worked fine locally with me with PyQT6, but support is probably experimental in napari at the moment (see napari/napari#5820)

[dstansby]

PySide2 doesn't have arm64 wheels: https://pypi.org/project/PySide2/#files. So I think for now we go with pyqt6.