Ikpy

* Drop Python 2 support
* PEP517 builds
* Support for type hints

Fix issue preventing use of IKPy on Python3.7 with PyPot: https://github.com/Phylliade/ikpy/issues/77

* Support for orientation
* Along one axis
* Along the full frame
* New JSON format which permits loading and serializing a chain from a JSON file
* Pre-created chains in the form of JSONs:
* Baxter: left-arm, right-arm, head, pedestal
* Poppy Torso
* Plotting functions greatly improved
* Utils to analyze an URDF file:
* Get a tree of links from the URDF file
* Plot the URDF tree

* Overall repository cleaning
* Add support for bounds in URDF files (42)

Complete rewrite of the library
- Use simple objects interface
- Expose API to customize behavior