Python 3 changed sys.stdout and sys.stderr to require Unicode

strings, but this doesn't work so well for binary data, which rbssh

writes. Instead, we need to use the buffer attribute of both of these.
This change updates rbssh to determine the right object to write to for

Python 2 and 3. It also disables warning output, which interferes with

the communication (and is triggered quite a lot by paramiko's

interaction with cryptography).

rbssh-based unit tests pass on Python 2 and 3 (when used with other

pending changes).