Example MQTT code, and simpletest fail when the MQTT client object's set_socket is called as MQTT.set_socket(socket, esp).
The file inclusion path (https://github.com/adafruit/Adafruit_CircuitPython_MiniMQTT/releases/tag/3.1.1) has to be modified in the examples to reflect the updated MiniMQTT library directory:
import adafruit_minimqtt.adafruit_minimqtt as MQTT