beets: 1.4.9 -> unstable-2020-12-22

The maintainer has stopped cutting releases[1]. Since the last release,
1.4.9, includes a dependency that is filled with hate speech[2] it's all
the more reason to package the unstable version and eliminate that
requirement.

Moreover a number of fixes, improvements, and features have landed
since.

[1]: https://github.com/beetbox/beets/issues/3625
[2]: https://github.com/NixOS/nixpkgs/pull/90504

Co-authored-by: Doron Behar <doron.behar@gmail.com>

pkgs/tools/audio/beets/replaygain-default-ffmpeg.patch

@@ -0,0 +1,26 @@
+diff --git i/beetsplug/replaygain.py w/beetsplug/replaygain.py
+index 9d6fa23c..c5800039 100644
+--- i/beetsplug/replaygain.py
++++ w/beetsplug/replaygain.py
+@@ -391,7 +391,7 @@ class FfmpegBackend(Backend):
+ 
+     def __init__(self, config, log):
+         super(FfmpegBackend, self).__init__(config, log)
+-        self._ffmpeg_path = "ffmpeg"
++        self._ffmpeg_path = "@ffmpeg@/bin/ffmpeg"
+ 
+         # check that ffmpeg is installed
+         try:
+@@ -1228,11 +1228,10 @@ class ReplayGainPlugin(BeetsPlugin):
+     def __init__(self):
+         super(ReplayGainPlugin, self).__init__()
+ 
+-        # default backend is 'command' for backward-compatibility.
+         self.config.add({
+             'overwrite': False,
+             'auto': True,
+-            'backend': u'command',
++            'backend': u'ffmpeg',
+             'threads': cpu_count(),
+             'parallel_on_import': False,
+             'per_disc': False,