Add Python 3.12 to tests

[nj1973]

Add 3.12 to the Python versions list PYTHON_VERSIONS in noxfile.py.

As part of this temporarily hardcode DEFAULT_PYTHON_VERSION to "3.12" and run all tests to ensure all engines work on this version. Depending on the outcome you need to decide whether to fix problems in this issue or raise issues for problems seen and put this back on the backlog.

Remember to return DEFAULT_PYTHON_VERSION to an earlier version than 3.12.