fix --list-frames-v2.2 option

Closes: #669

Author: Fat-Zer

mutagen/_tools/mid3v2.py

@@ -61,7 +61,7 @@ def list_frames(option, opt, value, parser):
 
 def list_frames_2_2(option, opt, value, parser):
     items = mutagen.id3.Frames_2_2.items()
-    items.sort()
+    sorted(items)
     for name, frame in items:
         print(u"    --%s    %s" % (name, frame.__doc__.split("\n")[0]))
     raise SystemExit