Allow deprecating stable functions/features

[encukou]

Old functions would need to be available forever, but they could be deprecated at build time.
Maybe they could even be removed for newer Py_LIMITED_API (but still be available at run time).

[encukou]

@vstinner, it seems you're interested in deprecating in the Limited API / Stable ABI.
Do you have a list of what you'd like to deprecated? Or just one thing you'd like to be gone?
It would be nice to do this with a relevant example :)

[vstinner]

In a perfect world, I would like to remove almost all functions of the Python C API :-) Only 2 or 3 functions should be enough for everyone :-D

In the meanwhile, I would like to remove PyEval_InitThreads(). Deprecated in 3.9, PyEval_InitThreads() now does nothing. See https://bugs.python.org/issue44117 and python/cpython#26070

In Python main branch:

void
PyEval_InitThreads(void)
{
    /* Do nothing: kept for backward compatibility */
}

[encukou]

That is an example of a function I'd not want to remove in abi3. Keeping it doesn't hurt anyone.

[encukou]

A candidate for removal is the old buffer protocol (see https://bugs.python.org/issue41103), but only after the new API is available in Limited ABI (https://bugs.python.org/issue45459):
- PyObject_AsCharBuffer
- PyObject_AsReadBuffer
- PyObject_AsWriteBuffer
- PyObject_CheckReadBuffer

[encukou]

Static types are another candidate: https://bugs.python.org/issue40601

[encukou]

PySlice_GetIndices/PySlice_GetIndicesEx are another candidate: https://discuss.python.org/t/is-pyslice-getindices-still-used/20778

PySequence_Fast is another candidate: python/cpython#91417

And more: Py_TPFLAGS_HAVE_FINALIZE, Py_TPFLAGS_HAVE_VERSION_TAG, PY_TIMEOUT_MAX, anything marked abi-only