docs: CallbackAPIVersion is not exported from module paho.mqtt.client

[GNITOAHC]

Running README sample will occer this error: "CallbackAPIVersion" is not exported from module "paho.mqtt.client"
It can be fixed by importing CallbackAPIVersion from paho.mqtt.enums

+from paho.mqtt.enums import CallbackAPIVersion

-mqttc = mqtt.Client(mqtt.CallbackAPIVersion.VERSION2)
+mqttc = mqtt.Client(CallbackAPIVersion.VERSION2)

[PierreF]

Thanks for your (long) patience. In order to merge your PR, could you sign ECA (so this check should become green)

[GNITOAHC]

Thanks! I’ve signed the ECA, and the check is green now.