Minimum python version to 3.11 TODO

- [ ] Remove [fix_hints_for_none_default](https://github.com/maread99/valimp/blob/800394e1db7dc04de0faf784ac2e5ddf9895394d/src/valimp/valimp.py#L764).
- [ ] Change `Parser` and `Coerce` classes to `dataclass.dataclasses` (different behaviour in python prior to 3.11 precludes these being dataclasses prior to 3.11)