Upgrade to Python3.x for ROS N-Turtle

[IanTheEngineer]

There is discussion in a REP for forcing a Python2.x -> Python 3.x transition during the N-Turtle release. See ros-infrastructure/rep#149 for details.

[dirk-thomas]

@sloretz FYI

[dirk-thomas]

@chris-smith Friendly ping since this package is required to bootstrap ROS Noetic which uses Python 3.

[IanTheEngineer]

@chris-smith - I'm slightly slammed with other work right now. How much work do you anticipate it will be to port this to Python 3.x?

[chris-smith]

As far as I'm aware it should work for python 3 - I was able to build messages with it using python 3 and ROS melodic. I assume we'll need to do a new release for the changes @sloretz made though.