Rewrite network testing without asyncore/asynchat

Running python3 -X dev -m chutney.TorNet with Python 3.11 prints:

/tmp/chutney/lib/chutney/Traffic.py:33: DeprecationWarning: The asyncore module is deprecated and will be removed in Python 3.12. The recommended replacement is asyncio
  import asyncore
/tmp/chutney/lib/chutney/Traffic.py:34: DeprecationWarning: The asynchat module is deprecated and will be removed in Python 3.12. The recommended replacement is asyncio
  import asynchat

According to https://peps.python.org/pep-0693/, Python 3.12 is scheduled for release on 2023-10-02.