fixup! Fix or ignore typing issues

Author: pganssle

src/audio_feeder/m4btools.py

@@ -289,7 +289,7 @@ def _extract_subset(
     if subset.end:
         subset_directives.extend(("-to", str(subset.end)))
 
-    cmd = [
+    cmd: Sequence[str] = [
         "ffmpeg",
         "-loglevel",
         "error",