bump Python to 3.12

[matthiasdiener]

The pytest failures do not seem to be related to:

• warnings: same numbers of warnings generated with 3.11
• memory: memory consumption seems to be about the same with 3.11 (see e.g. #1074)

Although generally slowdowns lead to CI cancelation in test_operators.py, testing that file separately seems to work fine (see https://github.com/illinois-ceesd/mirgecom/actions/runs/11484157110/job/31961509540).

Something about ctx_factory may be fishy - some test files use ctx_factory, some actx_factory.

Perhaps the order in which tests are run is relevant?

Or is it coverage testing?

Google sheets

[matthiasdiener]

Looks like coverage is our problem regarding 3.12, and it turns out 3.12 is even a bit faster than 3.11 with coverage disabled. Interestingly, there have been a few performance-related issues with coverage and 3.12, but all of these seem to hav been resolved in current package versions...

e.g., https://github.com/python/cpython/issues/119118 , https://github.com/nedbat/coveragepy/issues/1785