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.

analyzer/setup.py

@@ -34,10 +34,6 @@ setup(
         "pika>=1.0.0",
         "file-magic",
         "requests>=2.7.0",
-        "rgain3==1.1.1",
-        "PyGObject>=3.34.0",
-        # If this version is changed, it needs changing in the install script too
-        "pycairo==1.19.1",
     ],
     extras_require={
         "dev": [