Merge pull request #3191 from A5rocks/fix-parent-setter-type

Update type hint for the `CancelStatus.parent` setter

src/trio/_core/_run.py

@@ -394,7 +394,7 @@ def parent(self) -> CancelStatus | None:
         return self._parent
 
     @parent.setter
-    def parent(self, parent: CancelStatus) -> None:
+    def parent(self, parent: CancelStatus | None) -> None:
         if self._parent is not None:
             self._parent._children.remove(self)
         self._parent = parent