feat(analyzer): analyze replaygain using ffmpeg

- remove pycairo pip install
- fix py36 compatibility
- reraise when executable was not found

BREAKING CHANGE: The analyzer requires 'ffmpeg'. The 'rgain3' python package and it's system dependencies can be removed.

install

@@ -1023,9 +1023,6 @@ pip_cmd="$python_bin -m pip"
 
 verbose "\n * Installing necessary python services..."
 loudCmd "$pip_cmd install --upgrade setuptools~=58.0"
-# Required here because PyGObject requires it, but it is installed after PyGObject
-# when pip parses the setup.py file in analyzer
-loudCmd "$pip_cmd install pycairo==1.19.1"
 verbose "...Done"
 
 if [ ! -d /var/log/airtime ]; then