fix deprecation warning

Author: jjshoots

dm_control/manipulation/lift.py

@@ -67,7 +67,9 @@ def _add_vertex_sites(self, box_geom_or_site):
     """Add sites corresponding to the vertices of a box geom or site."""
     offsets = (
         (-half_length, half_length) for half_length in box_geom_or_site.size)
-    site_positions = np.vstack(itertools.product(*offsets))
+
+    site_positions = list(itertools.product(*offsets))
+    site_positions = np.vstack(site_positions)
     if box_geom_or_site.pos is not None:
       site_positions += box_geom_or_site.pos
     self._vertices = []