TYP: Series.name is Hashable - should it be str?
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [x] (optional) I have confirmed this bug exists on the master branch of pandas.

---

```python
import pandas as pd

s = pd.Series([1,2,3], name="foo")

reveal_type(s.name)

nam: str = s.name
```

mypy then reports:
```
seriesnam.py:5: note: Revealed type is "typing.Hashable*"
seriesnam.py:7: error: Incompatible types in assignment (expression has type "Hashable", variable has type "str")  [assignment]
```

It's natural to assume that the names of a Series are `str`

So why do we have it typed as `Hashable` ?

Seems like @simonjayhawkins changed this in https://github.com/pandas-dev/pandas/pull/39107
