Add platforms keyword to setup.py

[carver]

eg~

platforms=["unix", "linux", "osx"],

[davesque]

Wow, I have been duck-duck-going around for a while now and I can't for the life of me find anything that says what platforms are valid in that argument or how that metadata is used. I've even been poking around in the setuptools and distutils source code and still can't find anything.

The general meaning is clear but I thought it would be good to know exactly what a person is saying about their package if they claim they support unix.

[carver]

I didn't pull this from the docs, so it's possible that it's deprecated or even just wrong. I saw it in the libp2p setup.py, which I think pulled from the py-ecc one.

[carver]

Hm, looks like the better option is to add a classifier anyway: https://stackoverflow.com/a/34994245