There were several issues:
- json.dumps does not have an encoding parameter in Python 3.
- Forcing the json.dumps encoding to ASCII prevents any non-ASCII
  character in the message. To avoid having non-ASCII characters in the
  *dump*, the only needed parameter is ensure_ascii=True which is the
  default.
- To coerce a str to bytes in Python 3, an encoding must be provided
  even when it only contains ASCII characters.
  The solution is to use the encode() method which is available in both
  versions.