CLN: Optional[Hashable] in dict type hints
Minor clean-up.

```python
>>> from typing import Hashable
>>> isinstance(None, Hashable)
True
```

In old versions of mypy, mypy had a bug and didn't see this, but is has been fixed now.

There are many instances of `Optional[Hashable]` instead of just `Hashable` in the code base, other than in these dicts. I've assumed those should be left as-is for readability and not because it's strictly needed.
