Add ruff, black & isort configs

[MetRonnie]

I often find myself typing out some code that wouldn't pass flake8, then hitting "format selection" or "organise imports" in VSCode. I am using isort and ruff for this (having briefly used black before ruff).

I think it would be useful to define our configs for these tools on a project level so we can agree the style and avoid unnecessary diff resulting from this going forwards.

I've chosen defaults that I think best match our current prevailing style. If we want to haggle over this, make it brief!

To illustrate, I've ruffened/blackened and isorted pathutil.py as a small example.

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• No dependency changes
• No tests, changelog, docs needed
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[wxtim]

Seems reasonable.

[oliver-sanders]

Should be uncontroversial as it's basically just documenting what we do already.

We should probably sync this change to other repos in time 🤷 (I too have my own local .isort config to allow me to autoformat code selections in my test editor).