Add __all__ for most modules beginning with 't' (#7373)

stdlib/tkinter/dialog.pyi

@@ -1,6 +1,10 @@
+import sys
 from tkinter import Widget
 from typing import Any, Mapping
 
+if sys.version_info >= (3, 9):
+    __all__ = ["Dialog"]
+
 DIALOG_ICON: str
 
 class Dialog(Widget):