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/libretime_analyzer/utils.py

@@ -0,0 +1,24 @@
+from subprocess import PIPE, CalledProcessError, CompletedProcess, run
+
+from loguru import logger
+
+
+def run_(*args, **kwargs) -> CompletedProcess:
+    try:
+        return run(
+            args,
+            check=True,
+            stdout=PIPE,
+            stderr=PIPE,
+            universal_newlines=True,
+            **kwargs,
+        )
+
+    except OSError as exception:  # executable was not found
+        cmd = args[0]
+        logger.warning(f"Failed to run: {cmd} - {exception}. Is {cmd} installed?")
+        raise exception
+
+    except CalledProcessError as exception:  # returned an error code
+        logger.error(exception)
+        raise exception