improve requirements

[pattonw]

Separate requirements into different categories and automate installation

very low priority but would be nice to have.

If you build pyn5 from wheels, you only need numpy.
if you build pyn5 from source, you need the rust pipeline, setuptools_rust, and numpy
if you want to develop on pyn5, you need tox, flake8, etc. etc.

This can be done in setup.py install_requires field, I'm just not sure how to differentiate wheels and from source, and how to handle rust in the case of building from source