Decode a JSON decimal128 to bson.decimal128.Decimal128.

def _parse_canonical_decimal128(doc):
    """Decode a JSON decimal128 to bson.decimal128.Decimal128."""
    d_str = doc['$numberDecimal']
    if len(doc) != 1:
        raise TypeError('Bad $numberDecimal, extra field(s): %s' % (doc,))
    if not isinstance(d_str, string_type):
        raise TypeError('$numberDecimal must be string: %s' % (doc,))
    return Decimal128(d_str)