There was no big surprise. I wrote another short test program (mqtt-test.py) to subscribe to one of my moisture sensors. They simply provide an unsigned integer value. I merely have to get this value and define a threshold below which the soil is sufficiently wet or too dry if above and then set the GPIO accordingly. 

The sensor itself is presently not installed (still winter break for my irrigation system). So I provided typical values using MQTT-Explorer. It worked as expected.