Update paho-mqtt requirement from <2,>=1.5.0 to >=2.1.0,<3

Updates the requirements on [paho-mqtt](https://github.com/eclipse/paho.mqtt.python) to permit the latest version.
- [Release notes](https://github.com/eclipse/paho.mqtt.python/releases)
- [Changelog](https://github.com/eclipse/paho.mqtt.python/blob/master/ChangeLog.txt)
- [Commits](eclipse-paho/paho.mqtt.python@v1.5.0...v2.1.0)

---
updated-dependencies:
- dependency-name: paho-mqtt
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

requirements.txt

@@ -2,7 +2,7 @@ argparse>=1.4.0
 chump>=1.6.0
 construct~=2.9.43
 flake8
-paho_mqtt>=1.5.0,<2
+paho_mqtt>=2.1.0,<3
 
 pre-commit
 pushbullet.py>=0.11.0