Specify extension modules in setup.py.

[montebhoover]

We currently build native binaries, copy them into the python package root, and use setup.py to package them into the wheel. The best-practice way to do this is to specify the native extensions in setup.py as shown here: https://docs.python.org/3/extending/building.html#building-c-and-c-extensions-with-distutils.

I would estimate that this would take between 2-4 dev days for someone (like myself) who has never packaged native binaries with setup.py before.