boring stuff: bump minimum python version to 3.12

[qdbp]

other than the general reason of, "why not the latest?", I have one primary aim in suggesting this:

PEP-695, aka the "you can use generics without brainfuck-tier syntactic masochism" PEP

use of clean, composable protocols (see e.g. #31) ideally leans on generics, but pre 3.12 syntax really dirties things up.

fwiw I already use 3.12 in my env and everything seems to work fine