move DndSource to tkinter/dnd.pyi (#5588)

stdlib/tkinter/dnd.pyi

@@ -1,10 +1,12 @@
-from _typeshed.tkinter import DndSource
-from tkinter import Event, Misc, Tk
-from typing import ClassVar, Optional
+from tkinter import Event, Misc, Tk, Widget
+from typing import ClassVar, Optional, Protocol
+
+class _DndSource(Protocol):
+    def dnd_end(self, target: Optional[Widget], event: Optional[Event[Misc]]) -> None: ...
 
 class DndHandler:
     root: ClassVar[Optional[Tk]]
-    def __init__(self, source: DndSource, event: Event[Misc]) -> None: ...
+    def __init__(self, source: _DndSource, event: Event[Misc]) -> None: ...
     def cancel(self, event: Optional[Event[Misc]] = ...) -> None: ...
     def finish(self, event: Optional[Event[Misc]], commit: int = ...) -> None: ...
     def on_motion(self, event: Event[Misc]) -> None: ...