Move to using pyproject.toml and requirements.txt rather than setup.cfg+pyproject.toml+constraints.txt

[ThosRTanner]

The current system is confusing and requires constraints to be updated in multiple places. Moreover it renders the python caching a bit pointless as it's based off requirements.txt and - there isn't one.

Furthermore, the setup-python has its own support for pip caching and we should use that in preference to the explicit action-cache currently being used