Switch tox to using package dependencies

[digitaldogsbody]

Allowing tox to build the sdist means it deals with installing the dependencies in each environment. Combined this with manually specifying that the dev extras should be installed in the linter environment.

This should mean we no longer need to keep tox.ini up to date when our requirements change and so removes both workload/remembering and potential issues caused by differing versions when developing and testing (such as the Pydantic deep_copy issue) 🎉

Closes #98

[digitaldogsbody]

Ugh. And of course it runs fine locally and falls over on Github :(

Back to the drawing board...

[digitaldogsbody]

The problem was that Github Actions recently switched to Ubuntu 22.04 (actions/runner-images#6399), which has a newer version of libffi - this shouldn't normally be an issue as Python would get rebuilt when that library was updated, but our Python toolchain was cached in the action, meaning the Python it was using was compiled against the older version and hence throwing an error.

Changing the cache key by appending -u22 on the end was enough to force the rebuild / fix the problem.