convert: stringify `shlex` class argument The documentation is handwavy, but typeshed says this should be `str`[1]. I'm not sure if this is the correct encoding (vs `fsencode` or "latin1" like the tokens returned by the proxy class). While we're here, we can add a few more type hints that would have caused pytype to flag the problem. [1] https://github.com/python/typeshed/blob/6a9b53e719a139c2d6b41cf265ed0990cf438192/stdlib/shlex.pyi#L51