Add Python 3.12 to tests

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.