Update supported Python versions

[lindsay-stevens]

Current supported versions of Python 3.7, 3.8 and 3.9. Version 3.7 went EOL 2023-07-27. Version 3.8 is good until 2024-10-14. Version 3.12 was released on 2023-10-02.

So support for 3.7 should be dropped, and support for versions 3.10 and 3.11 should be added. In practical terms this means updating the GitHub actions workflow here. Worth seeing if 3.12 works, but if it doesn't then it's probably so new that few will be using it yet.

Also update the README accordingly.