gh-110481: Fix Py_SET_REFCNT() integer overflow (#112174)

If Py_NOGIL is defined and Py_SET_REFCNT() is called with a reference
count larger than UINT32_MAX, make the object immortal.

Set _Py_IMMORTAL_REFCNT constant type to Py_ssize_t to fix the
following compiler warning:

Include/internal/pycore_global_objects_fini_generated.h:14:24:
warning: comparison of integers of different signs: 'Py_ssize_t'
(aka 'long') and 'unsigned int' [-Wsign-compare]

    if (Py_REFCNT(obj) < _Py_IMMORTAL_REFCNT) {
        ~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~~~~~~

Doc/c-api/refcounting.rst

@@ -34,6 +34,9 @@ of Python objects.
 
    Set the object *o* reference counter to *refcnt*.
 
+   On :ref:`Python build with Free Threading <free-threading-build>`, if
+   *refcnt* is larger than ``UINT32_MAX``, the object is made :term:`immortal`.
+
    This function has no effect on :term:`immortal` objects.
 
    .. versionadded:: 3.9