You need the current git head of RobotPy to compile pynetworktables. RobotPy uses git submodules now, and if you don't check it out correctly then pynetworktables will not compile for you.
Code:
git clone https://github.com/robotpy/robotpy.git robotpy
cd robotpy
git submodule init
git submodule update
I've gotten it to work on Windows and Linux for Python 2 and 3.