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/PullRequestComment.py

@@ -34,10 +34,12 @@
 #                                                                              #
 ################################################################################
 
+from __future__ import absolute_import
 import github.GithubObject
 import github.NamedUser
 
-import Consts
+from . import Consts
+import six
 
 
 class PullRequestComment(github.GithubObject.CompletableGithubObject):
@@ -184,7 +186,7 @@ class PullRequestComment(github.GithubObject.CompletableGithubObject):
         :param body: string
         :rtype: None
         """
-        assert isinstance(body, (str, unicode)), body
+        assert isinstance(body, (str, six.text_type)), body
         post_parameters = {
             "body": body,
         }
@@ -216,7 +218,7 @@ class PullRequestComment(github.GithubObject.CompletableGithubObject):
         :param reaction_type: string
         :rtype: :class:`github.Reaction.Reaction`
         """
-        assert isinstance(reaction_type, (str, unicode)), "reaction type should be a string"
+        assert isinstance(reaction_type, (str, six.text_type)), "reaction type should be a string"
         assert reaction_type in ["+1", "-1", "laugh", "confused", "heart", "hooray"], \
             "Invalid reaction type (https://developer.github.com/v3/reactions/#reaction-types)"