[3.14] gh-127705: Move Py_INCREF_MORTAL() to the internal C API (GH-136178) (#136206)

gh-127705: Move Py_INCREF_MORTAL() to the internal C API (GH-136178)

Rename Py_INCREF_MORTAL() to _Py_INCREF_MORTAL() and move it to
pycore_object.h internal header.
(cherry picked from commit fa43a1e0f8caf00a15898fa719e31767c866bd90)

Co-authored-by: Victor Stinner <vstinner@python.org>