Conditional dependencies

[sloretz]

This makes the python_orocos_kdl package use conditional dependencies and ROS_PYTHON_VERSION for python-sip and python3-sip. This is necessary to use rosdep to install dependencies for this package since the master branch will support both Python 2 and Python 3. It was also required to delete the manifest.xml since it has an unconditional dependency on the rosdep key python-sip, which cannot be resolved on Ubuntu Focal.

See also http://wiki.ros.org/UsingPython3/SourceCodeChanges#Changing_dependencies_in_your_package.xml

required for #218

[meyerj]

Be aware that manifest.xml is not only used by older ROS versions, but also by other build tools used by the Orocos community, like Autoproj. We also parse it in rtt/UseOROCOS-RTT-helpers.cmake. The two specs used to be compatible a long time ago, but also Autoproj added new features that were not understood by the ROS tools.

The dual use of manifest.xml already caused some trouble in the past. I once proposed to prefer package.xml if it exists (ros-infrastructure/rosdep#353), but that was rejected back then. I noticed that the issue surfaced again recently (ros-infrastructure/rosdep#737).

In this particular case it should be fine to remove the manifest.xml as python_orocos_kdl is not used by Rock if I am not mistaken (@doudou?).

[doudou]

In this particular case it should be fine to remove the manifest.xml as python_orocos_kdl is not used by Rock if I am not mistaken (@doudou?).

I don't for sure (only use KDL C++). Not sure about the rest of the Rock users. I'll drop a line on the mailing list.

[meyerj]

I assume the new dependency <test_depend>python-psutil</test_depend> added in #216 would also require some special treatment for Python 3 now?

[sloretz]

Closing, no longer necessary for ROS Noetic (#218)