py3: ota_from_target_files.py: fix logic that broke

def MostPopularKey(d, default):

  """Given a dict, return the key corresponding to the largest

  value.  Returns 'default' if the dict is empty."""

  x = list(d.items())

  x = [(v, k) for (k, v) in d.items()]

  if not x:

    return default

  x.sort()