Add some missing type hints to tkinter (#6359)

Co-authored-by: Akuli <akuviljanen17@gmail.com>

stdlib/tkinter/simpledialog.pyi

@@ -4,7 +4,7 @@ from typing import Any
 class Dialog(Toplevel):
     def __init__(self, parent: Misc | None, title: str | None = ...) -> None: ...
     def body(self, master) -> None: ...
-    def buttonbox(self): ...
+    def buttonbox(self) -> None: ...
 
 class SimpleDialog:
     def __init__(