Test against python 3.12 (and maybe 3.13)

[lbdreyer]

I noticed the CI is testing against Python versions 3.9, 3.10 and 3.11.

It would be good to at least add Python 3.12 to this list as that is increasingly being adopted across the community.

I also include Python 3.13 as that version also is available through conda-forge although I'm not sure how much it is being used at present.