Port kdl_parser_py to ROS 2

[clalancette]

There are two packages in this repository: kdl_parser and kdl_parser_py. We've had a ROS 2 port for the first one for a long time, but we have never gotten around to porting kdl_parser_py to ROS 2. This is a good first task for someone to pick up. The rough steps as I see them:

1. Make sure the code in the kdl_parser_py package is up-to-date with the fixes that have landed on the noetic-devel branch.
2. Restructure the kdl_parser_py package to be a pure Python package and not use CMakeLists.txt at all (see https://github.com/ros2/demos/tree/2b94e70a06d9a0b37ba6b7d3405e5e2d1779e509/demo_nodes_py as one example of a pure Python package).
3. Remove the AMENT_IGNORE.
4. Add in the linters (again, see https://github.com/ros2/demos/tree/2b94e70a06d9a0b37ba6b7d3405e5e2d1779e509/demo_nodes_py/test for examples).
5. Port the tests to ROS 2.
6. Open the PR.