Tables: Fix get_window_extent for table to allow table to be added to bbox_extra_artists.

The get_window_extent is a method on the cells not the key of the cells.

Author: jenshnielsen

lib/matplotlib/table.py

@@ -279,7 +279,9 @@ def get_children(self):
 
     def get_window_extent(self, renderer):
         'Return the bounding box of the table in window coords'
-        boxes = [c.get_window_extent(renderer) for c in self._cells]
+        boxes = [cell.get_window_extent(renderer)
+                 for cell in self._cells.values()]
+
         return Bbox.union(boxes)
 
     def _do_cell_alignment(self):