Fix spawn_model for Python 3

[kartikmohta]

A recent fix (#449) seems to have broken the spawn_model script when using Python 3. This is caused due to the xml.etree.ElementTree.tostring function returning a bytestring in Python 3 while the following code expects a string.

Here is the error I received:

Traceback (most recent call last):
  File "/opt/ros/melodic/lib/gazebo_ros/spawn_model", line 233, in <module>
    exit_code = sm.run()
  File "/opt/ros/melodic/lib/gazebo_ros/spawn_model", line 179, in run
    self.args.gazebo_namespace)
  File "/opt/ros/melodic/lib/python3.7/site-packages/gazebo_ros/gazebo_interface.py", line 33, in spawn_urdf_model_client
    resp = spawn_urdf_model(model_name, model_xml, robot_namespace, initial_pose, reference_frame)
  File "/opt/ros/melodic/lib/python3.7/site-packages/rospy/impl/tcpros_service.py", line 439, in __call__
    return self.call(*args, **kwds)
  File "/opt/ros/melodic/lib/python3.7/site-packages/rospy/impl/tcpros_service.py", line 516, in call
    transport.send_message(request, self.seq)
  File "/opt/ros/melodic/lib/python3.7/site-packages/rospy/impl/tcpros_base.py", line 668, in send_message
    serialize_message(self.write_buff, seq, msg)
  File "/opt/ros/melodic/lib/python3.7/site-packages/rospy/msg.py", line 152, in serialize_message
    msg.serialize(b)
  File "/opt/ros/melodic/lib/python3.7/site-packages/gazebo_msgs/srv/_SpawnModel.py", line 103, in serialize
    _x = _x.encode('utf-8')
AttributeError: 'bytes' object has no attribute 'encode'

[timonegk]

Regarding the imminent end of life of python 2, could this pull request be merged @j-rivero?

[j-rivero]

@ros-pull-request-builder retest this please

[j-rivero]

Thanks Kartin for the PR and Timon for the ping at a good moment.