Fix return type of Image.reduce() in Pillow (#5946)

Image.reduce() should return an Image instead of None.

stubs/Pillow/PIL/Image.pyi

@@ -176,7 +176,7 @@ class Image:
         box: tuple[float, float, float, float] | None = ...,
         reducing_gap: float | None = ...,
     ) -> Image: ...
-    def reduce(self, factor: int | tuple[int, int] | list[int], box: _Box | None = ...) -> None: ...
+    def reduce(self, factor: int | tuple[int, int] | list[int], box: _Box | None = ...) -> Image: ...
     def rotate(
         self,
         angle: float,