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

diff --git a/Python/thread_pthread.h b/Python/thread_pthread.h
index 25f58d9..1f4f36d 100644
--- a/Python/thread_pthread.h
+++ b/Python/thread_pthread.h
@@ -302,7 +302,7 @@
     return (unsigned long) threadid;
 }
 
-void
+void _Py_NO_RETURN
 PyThread_exit_thread(void)
 {
     dprintf(("PyThread_exit_thread called\n"));