chore: remove nox

[joshdimanteto]

Description

This PR removes Nox from the repository. Because the project is now pinned to a single Python version, a multi-version test runner is no longer necessary
Changes:

• Removed noxfile.py.
• Updated CI workflows to run tests directly via pytest , flake8 and black.
• Updated ReadME

Testing Instructions

Add a set up instructions describing how the reviewer should test the code

• Review code
• Check GitHub Actions build
• If icatdb Generator Script Consistency Test CI job fails, is this because of a deliberate change made to the script to change generated data (which isn't actually a problem) or is here an underlying issue with the changes made?
• Review changes to test coverage
• Does this change mean a new patch, minor or major version should be made? If so, does one of the commit messages feature fix:, feat: or BREAKING CHANGE: so a release is automatically made via GitHub Actions upon merge?

Agile Board Tracking

closes #566

[patrick-austin]

Functionally it looks OK to me, but I think we might be calling poetry install repeatedly when we only need it once in the CI? Unless there's something too subtle for me to pick up on going on here.