Prefer setuptools with Python 3

[sloretz]

This is a follow up to ros/genmsg#90 (comment)

Currently catkin requires distutils and optionally supports setuptools; however, distutils for Python 3 requires a dependency on python3-distutils in Ubuntu Focal and Debian Buster. This PR makes catkin prefer setuptools and optionally support distutils when using Python 3.

• catkin itself uses setuptools in its own setup.py
• When interrogating another package's setup.py, don't fail if distutils.core can't be imported
• When using Python 3, add a buildtool_export_depend on setuptools since something like it must be available downstream
• Update documentation to use setuptools instead of distutils.core.

With this PR I think the effect on Python 2 ROS distros is python-setuptools would be installed where it wasn't before, but no downstream changes are required. The effect on Noetic is downstream packages depending on catkin either need to depend on python3-distutils or use setuptools instead.

[sloretz]

CI failure on Noetic appears to be due to CMake warning unrelated to this PR: http://build.ros.org/job/Npr__catkin__ubuntu_focal_amd64/4/warnings2Result/new/

[mikepurvis]

This change is causing a bit of chaos for us as the install directory has changed from lib/python3/dist-packages/catkin to lib/python3/dist-packages/catkin-0.8.0-py3.5.egg, with an lib/python3/dist-packages/easy-install.pth file generated that adds the egg path to sys.path on interpreter startup.

Part of the reason this is problematic is because we build in isolation with custom post-build merging, but even in a more conventional build scenario, there's a conflict over this file as it could be provided by multiple packages with varying contents.

Switching to setuptools makes sense, but I think we should be passing it the --single-version-externally-managed flag.

FYI @tspicer01

[ros-discourse]

This pull request has been mentioned on ROS Discourse. There might be relevant details there:

https://discourse.ros.org/t/ros-1-and-python-3-10s-deprecation-of-distutils-core/29834/15