convert: Split command in str form

...with yet another round-trip conversion. :/

beetsplug/convert.py

@@ -206,8 +206,15 @@ class ConvertPlugin(BeetsPlugin):
         source = decode_commandline_path(source)
         dest = decode_commandline_path(dest)
 
+        # Split the command using shell syntax. We need to pass the
+        # string through a `str` because, at least on some Python
+        # versions, shlex.split does not support bytes.
+        args = [
+            a.encode('utf8', 'surrogateescape')
+            for a in shlex.split(command.decode('utf8', 'surrogateescape'))
+        ]
+
         # Substitute $source and $dest in the argument list.
-        args = shlex.split(command)
         encode_cmd = []
         for i, arg in enumerate(args):
             args[i] = Template(arg).safe_substitute({