Pin paho-mqtt version < 2.0.0

[caronc]

Description:

Related issue (if applicable): n/a

MQTT v2 appears to be non-backwards-compatible with it's predecessor. This is causing the following error to appear in a lot of new services that require it.

Traceback (most recent call last):
File "/usr/local/lib/python3.8/site-packages/paho/mqtt/client.py", line 874, in del
self._reset_sockets()
File "/usr/local/lib/python3.8/site-packages/paho/mqtt/client.py", line 1133, in _reset_sockets
self._sock_close()
File "/usr/local/lib/python3.8/site-packages/paho/mqtt/client.py", line 1119, in _sock_close
if not self._sock:
AttributeError: 'Client' object has no attribute '_sock'

Apprise has been reported to be impacted here . Best work-around would be to make sure systems do not leverage the new paho-mqtt version; the official bug report is here: eclipse/paho-mqtt/814

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• No lint errors (use flake8)
• 100% test coverage

Testing

Anyone can help test this source code as follows:

# Create a virtual environment to work in as follows:
python3 -m venv apprise

# Change into our new directory
cd apprise

# Activate our virtual environment
source bin/activate

# Install the branch
pip install git+https://github.com/caronc/apprise.git@mqtt-avoid-v2-for-now

# Test out the changes with the following command:
apprise -t "Test Title" -b "Test Message" \
  "mqtt://credentials"

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (5de9b7b) 99.27% compared to head (d46f625) 99.27%.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1065   +/-   ##
=======================================
  Coverage   99.27%   99.27%           
=======================================
  Files         136      136           
  Lines       17738    17738           
  Branches     3618     3618           
=======================================
  Hits        17610    17610           
  Misses        119      119           
  Partials        9        9           

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