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/GitRelease.py

@@ -35,10 +35,12 @@
 #                                                                              #
 ################################################################################
 
+from __future__ import absolute_import
 from os.path import basename
 import github.GithubObject
 import github.NamedUser
 import github.GitReleaseAsset
+import six
 
 
 class GitRelease(github.GithubObject.CompletableGithubObject):
@@ -187,13 +189,13 @@ class GitRelease(github.GithubObject.CompletableGithubObject):
         :rtype: :class:`github.GitRelease.GitRelease`
         """
         assert tag_name is github.GithubObject.NotSet              \
-            or isinstance(tag_name, (str, unicode)),               \
+            or isinstance(tag_name, (str, six.text_type)),               \
             'tag_name must be a str/unicode object'
         assert target_commitish is github.GithubObject.NotSet      \
-            or isinstance(target_commitish, (str, unicode)),       \
+            or isinstance(target_commitish, (str, six.text_type)),       \
             'target_commitish must be a str/unicode object'
-        assert isinstance(name, (str, unicode)), name
-        assert isinstance(message, (str, unicode)), message
+        assert isinstance(name, (str, six.text_type)), name
+        assert isinstance(message, (str, six.text_type)), message
         assert isinstance(draft, bool), draft
         assert isinstance(prerelease, bool), prerelease
         if tag_name is github.GithubObject.NotSet:
@@ -221,9 +223,9 @@ class GitRelease(github.GithubObject.CompletableGithubObject):
         :calls: `POST https://<upload_url>/repos/:owner/:repo/releases/:release_id/assets <https://developer.github.com/v3/repos/releases/#upload-a-release-asset>`_
         :rtype: :class:`github.GitReleaseAsset.GitReleaseAsset`
         """
-        assert isinstance(path, (str, unicode)), path
-        assert isinstance(label, (str, unicode)), label
-        assert name is github.GithubObject.NotSet or isinstance(name, (str, unicode)), name
+        assert isinstance(path, (str, six.text_type)), path
+        assert isinstance(label, (str, six.text_type)), label
+        assert name is github.GithubObject.NotSet or isinstance(name, (str, six.text_type)), name
 
         post_parameters = {
             "label": label