In DraggableLegend, inherit DraggableBase.artist_picker.

... which is implemented to return self.ref_artist.contains(evt), and
DraggableLegend.ref_artist == self.legend, so all's good.

Can be checked by manually dragging a draggable legend:
```
plot([1, 2], label="foo"); legend().set_draggable(True)
```

Author: anntzer

lib/matplotlib/legend.py

@@ -70,9 +70,6 @@ def __init__(self, legend, use_blit=False, update="loc"):
         DraggableOffsetBox.__init__(self, legend, legend._legend_box,
                                     use_blit=use_blit)
 
-    def artist_picker(self, legend, evt):
-        return self.legend.contains(evt)
-
     def finalize_offset(self):
         update_method = cbook._check_getitem(
             {"loc": self._update_loc, "bbox": self._bbox_to_anchor},