Fix CI

[imjoehaines]

Goal

Various fixes for CI:

• The setup Python step is failing on Python 3.5, which is fixed with a workaround from https://github.com/actions/setup-python/issues/866#issuecomment-2109784318
• Tornado is incompatible with pytest 8.2, so we no longer allow that version to be installed when running the tornado tests (https://github.com/pytest-dev/pytest/issues/12263)
• The license audit is failing because of changes to its dockerfile, which have been fixed by bumping our python version and running it in a virtual environment
• A linting error in a test snuck through because it was covered up by other failures in CI

I've also bumped all the GH Actions actions versions to the latest releases