gh-106023: Update What's New in 3.13: _PyObject_FastCall()

[vstinner]

The function _PyObject_FastCall() was restored.

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📚 Documentation preview 📚: https://cpython-previews--117633.org.readthedocs.build/

• Issue: C API: What's the status of the half-private FASTCALL calling convention? #106023

[gvanrossum]

@vstinner Are you sure that function was restored? I don't see it in my repo (I see _PyObject_FastCallDict though).

[vstinner]

Oh, wait. I was confused between _PyObject_FastCall() and _PyObject_FastCallDict() :-(

I checked each function since I was really confused about these removals and changes.

Removed, private:

• _PyObject_FastCall()
• _PY_FASTCALL_SMALL_STACK: moved to the internal C API

Removed, internal:

• _PyObject_FastCallTstate()
• _PyObject_FastCallDictTstate(): renamed to _PyObject_VectorcallDictTstate()

Kept:

• METH_FASTCALL
• _PyStack_AsDict()

Kept, PEP 590:

• _PyObject_Vectorcall()
• _PyObject_VectorcallMethod()
• _PyVectorcall_Function()
• _PyObject_CallOneArg()
• _PyObject_CallMethodNoArgs()
• _PyObject_CallMethodOneArg()
• _PyObject_FastCallDict()
• _Py_TPFLAGS_HAVE_VECTORCALL

[vstinner]

I wrote PR gh-117676 to add again the What's New entry. Even if it's a private function, I prefer to document how to update affected code for this very specific function, since I know that it's used in the wild.