Use the highest bitrate stream regardless of codec

The get_audio_only function defaults to filtering for mp4 streams.
This seems arbitrary as ffmpeg has the capabilities for other codecs
too and higher quality may be achieved this way.

spotdl/download/downloader.py

@@ -180,7 +180,11 @@ async def download_song(self, songObj: SongObj) -> None:
         else:
             youtubeHandler = YouTube(songObj.get_youtube_link())
 
-        trackAudioStream = youtubeHandler.streams.get_audio_only()
+        trackAudioStream = youtubeHandler.streams.filter(only_audio=True).order_by('bitrate').last()
+
+        if not trackAudioStream:
+            print(f"Unable to get audio stream for \"{songObj.get_song_name()}\" from video \"{songObj.get_youtube_link()}\"")
+            return None
 
         downloadedFilePath = await self._download_from_youtube(convertedFileName, tempFolder,
                                                                trackAudioStream)