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

stdlib/tkinter/dnd.pyi

@@ -1,6 +1,10 @@
+import sys
 from tkinter import Event, Misc, Tk, Widget
 from typing import ClassVar, Protocol
 
+if sys.version_info >= (3, 9):
+    __all__ = ["dnd_start", "DndHandler"]
+
 class _DndSource(Protocol):
     def dnd_end(self, target: Widget | None, event: Event[Misc] | None) -> None: ...