[3.13] gh-120838: Add _PyThreadState_WHENCE_FINI (gh-121013)

We also add _PyThreadState_NewBound() and drop _PyThreadState_SetWhence().

This change only affects internal API.

(cherry picked from commit a905721b9c, AKA gh-121010)

Co-authored-by: Eric Snow <ericsnowcurrently@gmail.com>

Modules/_testinternalcapi.c

@@ -1585,8 +1585,8 @@ exec_interpreter(PyObject *self, PyObject *args, PyObject *kwargs)
     }
 
     PyObject *res = NULL;
-    PyThreadState *tstate = PyThreadState_New(interp);
-    _PyThreadState_SetWhence(tstate, _PyThreadState_WHENCE_EXEC);
+    PyThreadState *tstate =
+        _PyThreadState_NewBound(interp, _PyThreadState_WHENCE_EXEC);
 
     PyThreadState *save_tstate = PyThreadState_Swap(tstate);