Add support for python3

[mathias-luedtke]

We should introduce a flag PYTHON_VERSION (?) that switches to the python3 debians.

Some questions:

• Are additonal steps needed for python3 testing?
• Should we provide docker images for python3?

[130s]

Just a food for thought; rospkg runs tests on CI against multiple Python versions.

[mathias-luedtke]

master uses python3 for ROS2.
I don't know what's the status of ROS1 and python3, I guess it is targeted for noetic.

[130s]

I guess it is targeted for noetic.

Verified that seems to be the case.

• Melodic supports 2.7 (testing against 3.5 is recommended) from REP0003.
• A set of tutorial for transitioning to Python3 is recently started by OR on ROS wiki.

The tutorial above identifies a few areas that might need changes. Honestly I've never thought about Py3 transition, but I'm afraid there would be a lot of issues than I thought. Hope that won't be the case, but starting to provide CI environment earlier might be helpful for catching issues early? OR already opened up Jenkins job http://wiki.ros.org/UsingPython3/Contributing#Check_the_Noetic_CI_jobs for capturing issues early (although I'm struggling how to interpret its result).

[mathias-luedtke]

Melodic supports 2.7 (testing against 3.5 is recommended) from REP0003.

But how to test? And how to tell rosdep?

[mathias-luedtke]

But how to test? And how to tell rosdep?

I was able to figue it out, I guess ;)
#429 is based on http://wiki.ros.org/UsingPython3/BuildUsingPython3

I will have a look at the new Noetic CI jobs later

[mathias-luedtke]

Noetic support was merged: #499

[mathias-luedtke]

Python 3 is supporedt for ROS1 noetic and ROS2.
Proper support for ROS1 melodic is very hard to implement and most-likely not needed (anymore).