Drop Python 3.8 branches (#13776)

stdlib/tkinter/dialog.pyi

@@ -1,11 +1,9 @@
-import sys
 from _typeshed import Incomplete
 from collections.abc import Mapping
 from tkinter import Widget
 from typing import Any, Final
 
-if sys.version_info >= (3, 9):
-    __all__ = ["Dialog"]
+__all__ = ["Dialog"]
 
 DIALOG_ICON: Final = "questhead"