Unpinned `paho-mqtt` dependency in the `setup.py`

[Lenormju]

In the requirements.txt :

paho-mqtt~=1.1

which means roughly "any 1.1.x version".

While in the setup.py :

    install_requires  = ['robotframework', 'paho-mqtt'],

which means roughly "any version of paho-mqtt".

I am no expert at Python dependencies and packaging, but if my assumptions are correct, then it looks like an inconsistency.
Should paho-mqtt version be pinned to 1.1.x for robotframework-mqttlibrary to work ? Because that's what is implied by the requirements file.

---

I noticed this problem while investigating errors that started happening in my Robot tests after upgrading paho-mqtt from 1.5.1 to 1.6.1 :

•   File "/usr/local/lib/python3.8/dist-packages/paho/mqtt/client.py", line 640, in _sock_recv
      return self._sock.recv(bufsize)
  AttributeError: 'NoneType' object has no attribute 'recv'

• Error publishing: 4
• and some expected messages that were not received

I don't know yet if these problem come from this library or from paho itself, I still haven't found out.