Adding python 3.12 support

[w-k-jones]

Python 3.12 has been out for a few months now, and tobac dependencies have been updated to support it (a mamba install with python 3.12 works for me locally without problems). We should potentially add it to the matrix testing

In addition, we should consider when to deprecate python 3.8 as support was ended by many dependencies earlier this year. The v1.6 release may be a good time for this.