surface cleaning update for noetic

After these very minor changes, surface cleaning with the
following command worked for me. Stretch wiped the top of
a small nightstand.

$ roslaunch stretch_demos clean_surface.launch

stretch_demos/nodes/clean_surface

@@ -1,6 +1,6 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
-from __future__ import print_function
+#from __future__ import print_function
 
 from sensor_msgs.msg import JointState
 from geometry_msgs.msg import Twist