Check Python 3.13 compatibility

[ns-rse]

Python 3.13 release is imminent (due next week) and features the option to disable the "Global Interprator Lock" (GIL) which controls parallel processes.

The pytest-freethreaded extension allows checking of the impact of running testes with the GIL disabled.

We should test our code against this, may not require formal tests to be written (which would require additional images to be added to the test suite) but it will be important to know what, if any, this impact has.