Decode a JSON int64 to bson.int64.Int64.

def _parse_canonical_int64(doc):
    """Decode a JSON int64 to bson.int64.Int64."""
    l_str = doc['$numberLong']
    if len(doc) != 1:
        raise TypeError('Bad $numberLong, extra field(s): %s' % (doc,))
    return Int64(l_str)