[C API] Deprecate _Py_Identifier API

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I propose to deprecate the _Py_Identifier API.

• Deprecate private _PyObject_CallMethodId(), _PyObject_GetAttrId(), _PyUnicode_FromId().
• Remove immediately internal functions using _Py_Identifier.

See also the discussions:

• https://discuss.python.org/t/c-api-how-much-private-is-the-private-py-identifier-api/29190 (2023).
• https://mail.python.org/archives/list/python-dev@python.org/thread/DNMZAMB4M6RVR76RDZMUK2WRLI6KAAYS/ (2022)

Since 2022, CPython has been modified to replace _Py_IDENTIFIER() with _Py_STR(). Python no longer uses the _Py_IDENTIFIER() API anymore.

See also issue #141049 "Deprecate private C API functions".

Linked PRs

• gh-142217: Remove internal _Py_Identifier functions #142219
• gh-142217: Deprecate the private _Py_Identifier C API #142221