Decode a JSON MaxKey to bson.max_key.MaxKey.

def _parse_canonical_maxkey(doc):
    """Decode a JSON MaxKey to bson.max_key.MaxKey."""
    if doc['$maxKey'] is not 1:
        raise TypeError('$maxKey value must be 1: %s', (doc,))
    if len(doc) != 1:
        raise TypeError('Bad $minKey, extra field(s): %s' % (doc,))
    return MaxKey()