[BUG]: Python 3.12 support

[henryiii]

Required prerequisites

• Make sure you've read the documentation. Your issue may be addressed there.
• Search the issue tracker and Discussions to verify that this hasn't already been reported. +1 or comment there if it has.
• Consider asking first in the Gitter chat room or in a Discussion.

Problem description

@tacaswell reported that python/cpython#23205 from Python 3.12 breaks pybind11, which in turn breaks matplotlib. We should probably both fix this and see if we can reenable the dev tag for 3.12 testing (Might also be good to start playing with using PyType_FromMetaclass too).

Reproducible example code

No response

[tacaswell]

https://github.com/tacaswell/pybindbug has reproduction code + bisect script + bisect log.

[tacaswell]

and specifically it breaks https://github.com/contourpy/contourpy which breaks Matplotlib

[pwuertz]

Python 3.12 now supports global interpreter locks per interpreter (PEP 684). This, as far as I understand, requires more fine grained control over module initialization & cleanup (PEP 489, #2548).

Is this something pybind is eventually able to do, or is this a topic for a pybind reboot like nanobind?

[rwgk]

Is this something pybind is eventually able to do, or is this a topic for a pybind reboot like nanobind?

I don't think creating more clones of pybind11 is environmentally friendly for the world at large. (Unless we have a clean interop story, to avoid excessive copy-pasting from one namespace to another.)

Upshot: I think it'll be best to fit new features into pybind11, even though it obviously requires more thought than another reboot.