fix rqt_graph

fix rqt_graph

 In file "/opt/ros/noetic/lib/python3/dist-packages/rqt_graph/ros_graph.py", line 414, in _save_dot

change
handle.write(self._current_dotcode)
to
handle.write(self._current_dotcode.encode('utf-8'))

source:https://github.com/ros-visualization/rqt_graph/issues/85