Spurious test failure on Python 3.8

[JamesParrott]

Paho.mqtt's test does not allow for Python 3.8 stdlib's old API for Subprocess.Popen.returncode (it used to be able to be None)
Docs: https://docs.python.org/3.8/library/subprocess.html#subprocess.Popen.returncode

Unnecessarily strict test:

paho.mqtt.python/tests/lib/conftest.py

Line 74 in d200095

if proc.returncode != expected_returncode:

Example failure (tox job on last commit to master):

[PierreF]

I'm not sure this error is Python 3.8 specific. Even newer Python could return None in returncode (as Python 3.8, when process didn't yet exited).
So according to Python's documentation, we might have another issue, which is that processed isn't stopped correctly... well reading the code is will occur if process don't stop within 5 seconds:

    try:
        proc.wait(5)
    except subprocess.TimeoutExpired:
        proc.terminate() <-- here we wait nothing, so proc.returncode could very likely still be None - it totally depends on OS scheduling

I'll make this more robust.