Add test for Decimal128.
Add test for Decimal128.

mongodb-labs/python-bsonjs,ShaneHarvey/python-bsonjs,mongodb-labs/python-bsonjs,ShaneHarvey/python-bsonjs,mongodb-labs/python-bsonjs,mongodb-labs/python-bsonjs,ShaneHarvey/python-bsonjs,ShaneHarvey/python-bsonjs