umqtt: Compatibility with CPython

[amotl]

Dear Paul,

in the spirit of #49, [1] improves the umqtt module to be compatible with CPython. As I've learned, these kinds of changes are out of the scope of Pycopy to be incorporated into its module ecosystem, I just want to leave this notice as a reference to others looking for the same thing.

We needed these changes to support a CPython-based test harness for our Terkin Datalogger. The outcome where this is used can be inspected at [2].

With kind regards,
Andreas.

[1] master...daq-tools:improve-umqtt
[2] https://github.com/hiveeyes/terkin-datalogger/blob/23b8065a/test/test_wifi_mqtt.py