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!
To compare PullRequestReview objects, they need to have the 'url'
attribute because the CompletableGithubObject expects to be able to
inspect a '_url' attribute on objects when testing for equality.
This field is not currently listed in the Github documentation at
https://developer.github.com/v3/pulls/reviews/, but it's been included
since Github 2.9
(It even exists in the test data returned for the original
PullRequestReview PR)
Steve Kowalik