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!
The return value of MainClass.search_topics() was a paginated list of
Repository, which is incorrect. Add a Topic class to encapuslate what is
returned.
Fixes#929
Steve Kowalik