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.

Python/pystate.c

@@ -404,6 +404,7 @@ _Py_COMP_DIAG_POP
         &(runtime)->audit_hooks.mutex, \
         &(runtime)->allocators.mutex, \
         &(runtime)->_main_interpreter.types.mutex, \
+        &(runtime)->_main_interpreter.code_state.mutex, \
     }
 
 static void