Support for python3 with ROS1

[mathias-luedtke]

addresses #274

It looks like ROS1 must be rebuilt from scratch in order to support python3.
So I add a ROS_FROM_SCRATCH feature, which uses rosinstall_generator to fetch all released sources.

The latter came in handy add support for noetic (based on Debian Buster) as well.

[mathias-luedtke]

Works, but the auto-generation is not as powerful without ros-infrastructure/rosinstall_generator#25 being fixed.
As a fallback UPSTREAM_WORKSPACE should point to a (generated) rosinstall file.

[arturmiller]

I have tried this out, with one of my repositories, but I have not been successful. I tried to run ROS melodic with Python3 and got as error message: "env: ‘rosdep’: No such file or directory". Am I missing something? Here is the build report: https://travis-ci.org/arturmiller/pybind11_examples/jobs/628120545#L186
And here is my travis.yml: https://github.com/arturmiller/pybind11_examples/blob/fix/ci/.travis.yml

[mathias-luedtke]

@arturmiller:
If I recall it correctly, I got this error when a dependency pulled in python-catkin-pkg for some reason.
This would replace python3-catkin-pkg and therefore uninstall python3-rosdep.
I will have a look again.

[mathias-luedtke]

when a dependency pulled in python-catkin-pkg

In your case python-rospkg: https://travis-ci.org/arturmiller/pybind11_examples/jobs/628120545#L1154

[arturmiller]

Just now I see what it takes to support Python3. Your are right a dependency is not Python3 ready. This is the reason for my error. I was able to create a minimal example, which is tested with Melodic and Python3.
I think I found a bug. If the project has no dependencies to other ROS packages, rosinstall-generator throws an error. This is why I have included roscpp as dependency. I don't know if this should be discussed in this PR.
I think this PR is very Important and I'm glad you created it. Many maintainer want to test their projects with Python3 and don't know how. Python2 retires very soon.

[mathias-luedtke]

This is not needed anymore..
noetic is completely supported now and python 3 support for melodic seems to introduduce more problems than it solves.