PEP 803: Position abi3t as a new variant; use Py_TARGET_ABI3T to select it

[encukou]

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📚 Documentation preview 📚: https://pep-previews--4747.org.readthedocs.build/

[rgommers]

Thanks for this update @encukou. I will comment on DPO with a few thoughts, but while reading thought it'd be useful to notice the textual little things that could be improved - hence this review.

[encukou]

Thank you!

[encukou]

From more discussion, a new knob called Py_TARGET_ABI3T looks best.

[ngoldbaum]

I left a few more comments after my main one.

For what it's worth, I have a branch of PyO3 that compiles with _Py_OPAQUE_PYOBJECT set on the GIL-enabled build. If I also define Py_GIL_DISABLED, then I hit issues in PyO3 internals because PyCriticalSection isn't exposed. I could stub out the critical section wrappers like they're currently stubbed on the GIL-enabled build, but then the resulting extensions won't be safe on the free-threaded build.

The tests run, but I get a segfault early on that I'm still debugging - it may be a CPython bug but it may also be a bug in how I set up _Py_OPAQUE_PYOBJECT support in PyO3. I'm meeting with David Hewitt tomorrow morning to figure that out.

[encukou]

I plan to merge this early next week and update the discussion.
If you have a review in progress, let me know.