Run tests on py312 for more third-party projects

[AlexWaygood]

cattrs, mypy and pyanalyze all now support Python 3.12, and run tests on Python 3.12 in CI.

Also, get rid of a hack that's no longer necessary for running the cattrs tests, since cattrs now requires Python 3.8+

[JelleZijlstra]

There's something wrong with cattrs, apparently while trying to install the pendulum dependency but the error isn't very helpful.

[AlexWaygood]

There's something wrong with cattrs, apparently while trying to install the pendulum dependency but the error isn't very helpful.

It's odd: the cattrs test suite appears to be passing on py312 upstream: https://github.com/python-attrs/cattrs/actions/runs/6043137162/job/16399595172. But they don't appear to be installing pendulum upstream; not 100% sure why we are with our invocation of the test suite.

[AlexWaygood]

I've backed out the change for cattrs; we can leave it for now