Use Final in tkinter (#12545)

stdlib/tkinter/dialog.pyi

@@ -2,12 +2,12 @@ import sys
 from _typeshed import Incomplete
 from collections.abc import Mapping
 from tkinter import Widget
-from typing import Any
+from typing import Any, Final
 
 if sys.version_info >= (3, 9):
     __all__ = ["Dialog"]
 
-DIALOG_ICON: str
+DIALOG_ICON: Final = "questhead"
 
 class Dialog(Widget):
     widgetName: str