pass bytestringed names to fnmatch_all()

Due to `syspath`, fnmatch ends up comparing strings to bytes, so we must
make sure they are bytes. Fixes #2124

beets/util/__init__.py

@@ -267,7 +267,8 @@ def prune_dirs(path, root=None, clutter=('.DS_Store', 'Thumbs.db')):
             # Directory gone already.
             continue
         clutter = [bytestring_path(c) for c in clutter]
-        if fnmatch_all(os.listdir(directory), clutter):
+        match_paths = [bytestring_path(d) for d in os.listdir(directory)]
+        if fnmatch_all(match_paths, clutter):
             # Directory contains only clutter (or nothing).
             try:
                 shutil.rmtree(directory)