Add Python 3.12 to CI matrix.

[danielballan]

We intend to keep 3.8 (and maybe temporarily reinstate 3.7) up to v0.1.0 so that there is a stable version installable in older environments.

[danielballan]

We are waiting on numba to support 3.12. They have an RC out, so this should not be far off. Tracked at https://github.com/numba/numba/issues/9197

[danielballan]

The linked issue announced that a numba version supporting Python 3.12 has been released, and it looks like conda packages went up a little more than one day ago: https://anaconda.org/conda-forge/numba/files

Power-cycled to re-run CI

[danielballan]

Now the issue is blosc, which has not published a Python 3.12 wheel.

[danielballan]

Superseded by #719