Modernise tooling and fix CI failing

[DMRobertson]

Commitwise reviewable.

I think the only necessary changes were

• Fix a unit test failure
• Dump flake8 for ruff

In particular, moving to pyproject.toml was my first attempt/guess to fix the CI errors, but I don't think it was strictly necessary.

[dklimpel]

Failing flake8 looks like this: https://github.com/python/importlib_metadata/issues/406

:(

[DMRobertson]

This is an exc

Failing flake8 looks like this: python/importlib_metadata#406

:(

Good spot, thank you.

This is an excellent excuse for us to move to ruff instead of flake8.

[DMRobertson]

There is one unit test failing. I can't obviously see why it's failing.

[dklimpel]

Fix an unintended rename in #333

Sorry.

[DMRobertson]

Fix an unintended rename in #333

Sorry.

No worries. It's easily done, and was our fault for having broken CI.