gh-118527: Intern code consts in free-threaded build (#118667)

We already intern and immortalize most string constants. In the
free-threaded build, other constants can be a source of reference count
contention because they are shared by all threads running the same code
objects.

Objects/setobject.c

@@ -2621,6 +2621,12 @@ PySet_Clear(PyObject *set)
     return 0;
 }
 
+void
+_PySet_ClearInternal(PySetObject *so)
+{
+    (void)set_clear_internal(so);
+}
+
 int
 PySet_Contains(PyObject *anyset, PyObject *key)
 {