Mqttwarn

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- [http] Rename ``http.py`` module to ``http_urllib.py`` to reduce conflicts with stdlib.
For backward compatibility reasons, it is still available by the same name, so no
configurations will break. Thanks, Morten!

=================

- [build] Fix unwanted cache hits when automatically building Docker images. Thanks, Gergő!
- [core] Respect relative path names within configuration file. This applies
to both the function file as well as module files.

=================
- [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, Gergő!
- [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.

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- [telegram] Fix encoding of data for python3. Thanks, Ben!

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- [xbmc] Fix encoding of data for python3. Thanks, Ben!
- [hipchat, ionic, pushsafer] Fix charset encoding. Thanks, JP!
- [telegram] Add compatibility for both Python 2 and Python 3. Thanks, clach04!
- Add new service for Chromecast TTS. Thanks, clach04!
- Add example for Amazon Alexa Say/Announce. Thanks, clach04!
- Improve documentation. Thanks, clach04!
- Fix Apprise service by explicitly using legacy/synchronous mode.
- Add Python 3.9 support

=================
- Use ``allow_dirty = False`` within ``.bumpversion.cfg``
- Use Python3 to create virtualenv
- Bump version numbers for release tools packages
- Add external plugin module loading. Thanks, psyciknz!
- Replaced iothub service with azure-iot, just using MQTT. Thanks, Morten!