Decode a JSON MinKey to bson.min_key.MinKey.

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