Instructions in CONTRIBUTING no longer work with tox>=4

Tox version>=4.0 is invoked differently (it's a complete rewrite from 3.x); the instructions on running unit tests in CONTRIBUTING.md no longer work.

Presumably either they should be updated to work with the current version of tox, or should be amended to specify `pip install 'tox<4'` to stay on the older version.


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This is a 📕 documentation issue
