First crack at Python 3 compatibility

Mostly we just use mininet.util.pexpect so that we can use
unicode strings without changing calls to pexpect.spawn()

examples/test/test_simpleperf.py

@@ -5,7 +5,7 @@ Test for simpleperf.py
 """
 
 import unittest
-import pexpect
+from mininet.util import pexpect
 import sys
 from mininet.log import setLogLevel