Remove "from __future__ import print"

This was well-intentioned, but it causes more trouble
than it's worth.

Fixes #652

examples/sshd.py

@@ -16,7 +16,6 @@ demonstrates:
 - running server processes (sshd in this case) on hosts
 """
 
-from __future__ import print_function
 import sys
 
 from mininet.net import Mininet