KemenyYoungVoting crashes

[Mate]

KemenyYoungVoting only works if for every pair of candidates A and B there is at least one voter who prefers A to B.
This is the case for the example under
https://pythonhosted.org/ballotbox/docs/methods/singlewinner/preferential.html

You can trigger the problem by having a single vote only:

>>> from ballotbox.singlewinner.preferential import KemenyYoungVoting
>>> bb = BallotBox(method=KemenyYoungVoting)
>>> preference = {"Memphis": 1, "Nashville": 2, "Chattanooga": 3, "Knoxville": 4}
>>> bb.add_votes(preference, 1)
>>> bb.get_winner()

The fix is to change this:

    def get_ranks(self):
        ranks = []
        for possibility in itertools.permutations(self.preference_options):
            rank = 0
            for index, option1 in enumerate(possibility[:-1]):
                for option2 in possibility[index + 1:]:
                    key = "%s > %s" % (option1, option2)
                    rank += self.lookup[key] ############### CHANGE THIS LINE
            ranks.append((rank, possibility))
        return sorted(ranks, reverse=True)

to this:

    def get_ranks(self):
        ranks = []
        for possibility in itertools.permutations(self.preference_options):
            rank = 0
            for index, option1 in enumerate(possibility[:-1]):
                for option2 in possibility[index + 1:]:
                    key = "%s > %s" % (option1, option2)
                    rank += self.lookup.get(key, 0) ############### TO THIS
            ranks.append((rank, possibility))
        return sorted(ranks, reverse=True)