Rename MQTTSubscribe.py to mqttsubscribe.py

[bellrichm]

This will align (correct) the name with python conventions - hopefully making it easier for others to support.
This will be a breaking change, so needs to be major release - perhaps V4.
To ease transition, a temporary wrapper with deprecated warnings should be put in place. For example MQTTSubscribe.py 'calls' mqttsubscribe.py.

[bellrichm]

I’ve been able to write a compatibility shim and have the install process update existing configs.
So… I think it is safe to target 3.2