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/Milestone.py

@@ -30,6 +30,7 @@
 #                                                                              #
 ################################################################################
 
+from __future__ import absolute_import
 import datetime
 
 import github.GithubObject
@@ -37,6 +38,7 @@ import github.PaginatedList
 
 import github.NamedUser
 import github.Label
+import six
 
 
 class Milestone(github.GithubObject.CompletableGithubObject):
@@ -170,9 +172,9 @@ class Milestone(github.GithubObject.CompletableGithubObject):
         :param due_on: date
         :rtype: None
         """
-        assert isinstance(title, (str, unicode)), title
-        assert state is github.GithubObject.NotSet or isinstance(state, (str, unicode)), state
-        assert description is github.GithubObject.NotSet or isinstance(description, (str, unicode)), description
+        assert isinstance(title, (str, six.text_type)), title
+        assert state is github.GithubObject.NotSet or isinstance(state, (str, six.text_type)), state
+        assert description is github.GithubObject.NotSet or isinstance(description, (str, six.text_type)), description
         assert due_on is github.GithubObject.NotSet or isinstance(due_on, datetime.date), due_on
         post_parameters = {
             "title": title,