Add @cpython_only to test_PyThreadState_SetAsyncExc

which is stated by the document

Author: youknowone

Lib/test/test_threading.py

@@ -323,6 +323,7 @@ def f(mutex):
 
     # PyThreadState_SetAsyncExc() is a CPython-only gimmick, not (currently)
     # exposed at the Python level.  This test relies on ctypes to get at it.
+    @cpython_only
     def test_PyThreadState_SetAsyncExc(self):
         ctypes = import_module("ctypes")