Changes waitid to return waitid_result | None (#10589)

Fixes #10564

Author: gotyaoi

stdlib/os/__init__.pyi

@@ -912,7 +912,7 @@ else:
             @property
             def si_code(self) -> int: ...
 
-        def waitid(__idtype: int, __ident: int, __options: int) -> waitid_result: ...
+        def waitid(__idtype: int, __ident: int, __options: int) -> waitid_result | None: ...
 
     def wait3(options: int) -> tuple[int, int, Any]: ...
     def wait4(pid: int, options: int) -> tuple[int, int, Any]: ...