My tip for this is Node.js and some stream processing lib like Highland. You can get ridiculous IO parallelism with a very little code and a nice API.

Python just scales terribly, no matter if you use multi-process or not. Java can get pretty good perf, but you'll need some libs or quite a bit of code to get nonblocking IO sending working well, or you're going to eat huge amounts of resources for moderate returns.

Node really excels at this use case. You can saturate the lines pretty easily.