Fixup / workaround: provide tox 'install_command' for the 'py' unit test environment name

[jayaddison]

Attempts to resolve #698 (I've tested this locally, but not on an OSX platform, so I'm uncertain whether tests will pass across all platforms).

Bases upon #700 so that both install command configurations are consistent (and both are consistent with the tox default).

[jayaddison]

Ok - the py-darwin install_command evaluated to python -I -m pip install -r /Users/runner/work/recipe-scrapers/recipe-scrapers/requirements-dev.txt python -I -m pip install --only-binary=lxml '{opts}' '{packages}' (with the pip install string appearing twice instead of once).

That seems odd.

[jayaddison]

There could be ways to adjust the tox.ini file to make this work with tox v4.0.8 and above. An alternative could be to pin to earlier versions of tox. I'm undecided yet about how to proceed -- either way, there could be another bug report to file here related to the duplication-of-install-command-contents.

[jayaddison]

To unblock continuous integration testing, pinning tox to <4 may be a shorter-term solution.

[jayaddison]

Superseded by #704.