V3.0.3 corrects the processing of MQTT username/password for connecting to authenticating brokers. Code revisions are noted as #3.0.3.
V3.0.2 was a minor update that removed Paho-MQTT callback V1 support and uses the V2 callback protocol instead. The next major release of Paho-MQTT will not support V1 callback protocol at all, and current Mac OSX (Tahoe) and Raspberry Pi OS (Trixie) distributions now both support V2 callbacks.
If you need to use V1 callback protocol because your OS doesn't offer the current Paho-MQTT release, invoke the prior version, DNT.3.0.1. Check your Paho_MQTT version with check_paho_version.