Add missing '-> None' to all __init__ methods.

builtins/3/_io.pyi

@@ -5,7 +5,7 @@
 from typing import Any
 
 class _IOBase:
-    def __init__(self, *args, **kwargs): ...
+    def __init__(self, *args, **kwargs) -> None: ...
     @property
     def closed(self): ...
     def close(self): ...