Fix named split sorting and remove unnecessary casting
_The documentation is not available anymore as the PR was closed or merged._ hahaha what a timing, I added my comment right after you merged x)

you can ignore my (nit), it's fine Sorry, just too sync... :sweat_smile: 
This PR:
- makes `NamedSplit` sortable: so that `sorted()` can be called on them
- removes unnecessary `sorted()` on `dict.keys()`: `dict_keys` view is already like a `set`
- removes unnecessary casting of `NamedSplit` to `str`