Migrate to tox 4.0

[Saransh-cpp]

Description

• A list of breaking changes - https://tox--2601.org.readthedocs.build/en/2601/faq.html#breaking-changes-in-tox-4.
• The relevant issue about env variables not being visible - New version of tox does not see environment variables tox-dev/tox#2659 (macOS tests are probably failing because of this).

Fixes # (issue)

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s. If necessary, also add to the list of breaking changes.

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist:

• No style issues: $ flake8
• All tests pass: $ python run-tests.py --unit
• The documentation builds: $ cd docs and then $ make clean; make html

You can run all three at once, using $ python run-tests.py --quick.

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[Saransh-cpp]

macOS looks okay now

[Saransh-cpp]

TODO:

• Dig into isolated_build more, which is most probably causing the ubuntu tests to fail.
• Update "tox<4" occurrences with tox

[valentinsulzer]

@Saransh-cpp feel free to change to nox or other if you're more comfortable with it. tox is nice but none of us really know how to maintain it

[brosaplanella]

Should we close this PR as we have now #3005 open?

[Saransh-cpp]

Yes!