Python 3.12 support

[benjeffery]

Python 3.12 has been released. The good news is that tskit builds and almost all tests pass. The failing ones are those that depend on numba via lshmm as numba for 3.12 has not yet been released. As this will be an issue for every Python release, I suggest we make those tests optional and raise a warning instead of running the tests.

[jeromekelleher]

Warnings tend to get ignored and forgotten about

It should be easy enough to, e.g., mark the test module as a skip if python==312 or something? We're going to rely on numba more and more, I'm afraid

[benjeffery]

Completed in #2964