Update Python version from 3.10

It is now less than a year until Python 3.10 is EoL so we should probably start thinking about what version we should like to move to.

https://devguide.python.org/versions/

I wouldn't anticipate significant code changes (if any) with this. I think it's more likely that any disruption would come from the knock on version changes in our dependencies.

Start with the Python versions page linked in the issue and inventory the repository's current Python and dependency versions. Done means agreeing on a target newer than 3.10 and updating the relevant version and dependency declarations while checking for disruption from dependency changes.

Written by the indexing model from the issue text.