bpo-36475: Make PyThread_exit_thread with _Py_NO_RETURN (GH-13068)

diff --git a/Include/pythread.h b/Include/pythread.h
index eb61033..bc1d92c 100644
--- a/Include/pythread.h
+++ b/Include/pythread.h
@@ -23,7 +23,7 @@
 
 PyAPI_FUNC(void) PyThread_init_thread(void);
 PyAPI_FUNC(unsigned long) PyThread_start_new_thread(void (*)(void *), void *);
-PyAPI_FUNC(void) PyThread_exit_thread(void);
+PyAPI_FUNC(void) _Py_NO_RETURN PyThread_exit_thread(void);
 PyAPI_FUNC(unsigned long) PyThread_get_thread_ident(void);
 
 PyAPI_FUNC(PyThread_type_lock) PyThread_allocate_lock(void);