Thanks. I'm not sure about the precise differences between six and future, but it seems like future is the preferred method, at least according to the official Python docs. That page mentions that six is able to provide Python 2.5 support, while future does not.
I removed the future stuff because it wasn't working on Python 2 and I wanted to eliminate the dependency on future. When I get back to it, I'll make a branch for further testing with getting 2 & 3 working on the same code.
RE: A Twisted-based asynchronous JSON-RPC STEEM library for Python