Update pymongo to 3.9.0 and remove python2 support
Changes
- Support for MongoDB 4.2 sharded transactions.
- Initial support for client side field level encyption.
- The retryWrites URI option now defaults to True.
- Support for retryable reads and the retryReads URI option which is enabled by default.
- Support zstandard for wire protocol compression.
- Support for periodically polling DNS SRV records to update the mongos proxy list without having to change client configuration.