The Crossbar broker and Autobahn client libraries (in multiple languages) implement a fully-specified and multi-language system that is similar to MQTT (called WAMP, the Web Application Messaging Protocol). This gives you cross-process/computer Pub/Sub and RPC over many client languages.

The Crossbar broker (implemented in Python using Twisted) interoperates with MQTT as well.

Note that it doesn't cover *every* MQTT QoS level, but bare MQTT brokers don't either.