If I were doing what you were doing, I would just take the NetworkTables.py file only and copy it to where your script was, and import that. To do what you're doing, you don't need any of the rest of the site-packages contents -- some of that requires a functioning wpilib, which is a binary module compiled for the cRio. I do have a 'fake' wpilib that will run on a PC (which is still in flux, but works) that contains the NetworkTables in it (
https://github.com/robotpy/fake-wpilib)... of course, it's the same file, so nothing different there.
However, looking more closely, I don't know if the RobotPy implementation supports being the client. It does provide the server implementation (which is on the robot itself), but it may not provide the client implementation. You could probably use it as a starting point for a client implementation however, if it isn't already implemented. I'd accept patches for that if you get it working.
Looking at the ConnectionManager, it appears to have a mechanism to not be the server. And you can create a connection manually.. so perhaps that would be what you need to do.
I've used the NetworkTables successfully on a PC to be the server side, but I haven't tried using it as the client. Seems like it might be possible though.