Update test config

[mikemanger]

Just tidying up the tox config:

• Update GitHub actions to fix warnings
• Update workflow to test against Python 3.11, 3.12 and 3.13
• Update classifiers
• Move tox config to pyproject.toml
• Don't lock version requirement for tox config as most projects don't support python 3.7 and 3.13 (we can narrow this by either doing minimal/max or by dropping support for 3.7 and 3.8 but thought I'd leave that for another task 🙂)
• Use tox-gh for workflow

[sklarsa]

lgtm, thank you for updating this!