[WIP] bpo-42294: Add Py_SetRef() and Py_XSetRef()

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Add Py_SetRef() and Py_XSetRef() functions to set a strong reference
to a new object without creating a temporary dangling pointer.

Rewrite the Py_SETREF() and Py_XSETREF() macros using respectively
Py_SetRef() and Py_XSetRef() functions.

Rewrite the Py_CLEAR() macro using the Py_XSetRef() public.

https://bugs.python.org/issue42294

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This change exposes Py_SETREF() and Py_XSETREF() macros as regular functions to allow using them in extension modules which cannot use the C preprocessor, like extensions written in Rust. It also adds Py_SetRef() and Py_XSetRef() as regular functions part of the limited C API and the stable ABI, whereas Py_SETREF() and Py_XSETREF() are excluded from the limited C API.

The Py_SETREF() and Py_XSETREF() macros were added in 2016 by @serhiy-storchaka in bpo-26200 with the commit 57a01d3.

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cc @methane @serhiy-storchaka

[methane]

I am not sure about this change really helps Rust, PyPy, HPy, etc.
Is this proposal based on some discussion in some forum?

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I am not sure about this change really helps Rust, PyPy, HPy, etc.
Is this proposal based on some discussion in some forum?

There was no discussion about that. I prefer to convert this PR to a draft until I have more time to explain the overall rationale for https://bugs.python.org/issue42294

Also, Serhiy wrote that Py_SETREF() had some flaws, I didn't know that. I will have to dig into old discussions.

[github-actions]

This PR is stale because it has been open for 30 days with no activity. Remove stale label or comment or this will be closed in 5 days

[github-actions]

This PR is stale because it has been open for 30 days with no activity.

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I close this PR since I failed to prove that it's useful for Rust, PyPy or HPy.