This updates the python version with poetry to match the python version used in the Dockerfile.

When the PlayState is None we assume that something went wrong during preloading. If it is None for more than 15 seconds we log an error and stop the preloading. Maybe this logic should be moved into the PlayState itself, which then could set the status to TIMEOUT.

Audio channel naming

See merge request !48

Refactor: add error handling for unknown channel type

See merge request !47

With the new liquidsoap versions all sources have a unique id, this id is used to create the namespace for the corresponding telnet commands. Since we alter the sources in liquidsoap multiple times we have a miss match in the namespace of some telnet commands. `mixer.inputs` returns a list of sources which may contain a id. This id is concatenated with the name of the source. For example `in_stream_0.1` is a valid response from `mixer.inputs`.  We therefore drop everything after the first `.`.