Fix thread termination behavior for Python <= 3.7.3

[0snap]

📔 Description

This PR fixes a bug that was introduced with #61

This bug caused Threat Bus to terminate directly after startup in Python 3.7.3 and earlier. A detailed description of the issue can be found in this Stackoverflow post.

📝 Checklist

• All user-facing changes have changelog entries.
  Not needed: This fixes an unreleased bug and a Changelog entry for the desired behavior already exists.
• The changes are reflected on docs.tenzir.com/threatbus, if necessary.
• The PR description contains instructions for the reviewer, if necessary.

🎯 Review Instructions

Commit-by-commit

Test this as follows:

• you need two Python installations, 3.7 <= Python <= 3.7.3 and Python >= 3.7.4
• two virtualenvs
• run make dev-mode in both envs
• craft a config that activates all plugins, see config.yaml.example
• start Threat Bus: venv/bin/threatbus -c config.yaml in each env (one at a time, else they try to use the same ports)
• wait a sec. it should keep running and not terminate on its own. send a few messages if you like.
• terminate it via ctrl+c
• it should gracefully stop all plugins, independent on the used Python version.

[0snap]

@satta This PR makes Threat Bus work with Python 3.7.3, the default Python version installed with Debian Buster.

[satta]

Very nice, thanks!

[mavam]

I did not run these, but if someone can confirm that these work, this can be merged.

Just to be clear: I'm green-lighting the functionality that stop and join can only be called if the thread is alive.