Use Python 3.12 for asyncio CI

[erikjohnston]

Hopefully to make it faster?

[erikjohnston]

Trying using uvloop has (fairly predictably) had no effect.

[clokep]

Note that the reactor tick timing stuff won't work with uvloop. I think I tried to support it briefly and then decided we probably weren't going to use it...

[erikjohnston]

Useful context! Was mainly wondering if using a different event loop would change how fast it'd run, but alas not. I guess there is just some extra overhead maybe?

[clokep]

I think the asyncio reactor you end up doing some bookkeeping twice, which is annoying. I'm kind of surprised it is that much of a difference though, I wish we had a good benchmark for running this through instead of sytest.