Convert stub: Python 3 compatibility

Important for systems where `python` is 3.x, like Arch, even when beets itself
is running on Python 2.

test/rsrc/convert_stub.py

@@ -10,6 +10,10 @@ import sys
 def convert(in_file, out_file, tag):
     """Copy `in_file` to `out_file` and append the string `tag`.
     """
+    # On Python 3, encode the tag argument as bytes.
+    if not isinstance(tag, bytes):
+        tag = tag.encode('utf8')
+
     with open(out_file, 'wb') as out_f:
         with open(in_file, 'rb') as in_f:
             out_f.write(in_f.read())