tox configuration requires tox version 4 about python-tooling

[samcunliffe]

Thanks a lot for trying it out!

@paddyroddy, @p-j-smith, what do you think? I'd be up for pinning tox v4 and deleting all the legacy stuff.
We get tox 4.3 on the GitHub runners. And newer is... gooder?

Is there some place where v3 is kicking around? (my 🍎 brew and pip both give me 4.6).

[p-j-smith]

I'd be up for pinning tox v4 and deleting all the legacy stuff.

yeah definitely, newer is much gooder. tox v4 requires Python>=3.7, but I think that's fine?

[matt-graham]

Is there some place where v3 is kicking around? (my 🍎 brew and pip both give me 4.6).

Good question. I'm not sure why the local conda environment I was using picked the older version it did (v3.27.1), but guessing one of the upstream dependencies when I created environment was pinning tox<4 as v3.27.1 seems to be the latest v3.x release for which a conda-forge package is available, though conda happily updated to v4.6.4 in the same environment when I noticed the above issue so doesn't seem to be anything restricting now. Don't know of any good reasons for not using tox v4 so seems safe to pin, and at least this would give a loud error when trying to resolve dependencies if another package is restricting the tox version.

[paddyroddy]

Some thoughts:

• tox==4.0.0 came out on 07/12/2022. I hadn't considered that people might even be using earlier versions. Makes sense to pin (I try to refrain from pinning too much). Personally, I don't think we should be supporting <4.
• The legacy_tox_ini sounds like its legacy, but it is because pyproject.toml doesn't fully support toml for tox. See here why we do it this way https://tox.wiki/en/4.6.4/config.html#pyproject-toml
• IMO we don't need to pin the actions. I guess we could, but then what happens when 5 is released and we forget we've pinned it.

I'll raise a PR to fix this - which is just by pinning in pyproject.toml. Thanks for spotting @matt-graham!