Switch to using six (#1189)

With the Python 2.7 deadline fast approaching, modernize the codebase
making use of the modernize module to switch to using six, as well as
other upcoming features, such as absolute imports . Stop using 2to3
for Travis, yay!

github/Event.py

@@ -30,6 +30,7 @@
 #                                                                              #
 ################################################################################
 
+from __future__ import absolute_import
 import github.GithubObject
 
 import github.Organization