0.21.0

• [xmpp] Add slixmpp plugin and documentation. Thanks, Remi!
• [mysql] Fix unicode vs byte issue for Python 3. Thanks, Filip!
• [http] Fix to send whole message. Thanks, Gergo!
• [slack] Replace slacker with slack-sdk. Thanks, mcdamo!
• [mysql] Fix specified port bug. Thanks, Hyman!
• [launch] Add new service launch. Flexible arguments for command,
  responds with MQTT publish. Thanks, Jörg!
• [core] Remove "os.chdir" as it is apparently not needed anymore. Thanks, Dan!
• [ci] Run tests on Python 3.9, remove testing on Python 3.5
• [core] Load service plugins from both modules and files