TYP: sort_index
Tested the overloads with mypy and pyright:

```py
import pandas as pd

def bool_() -> bool:
    ...

frame = pd.DataFrame()
reveal_type(frame.sort_index()) # DataFrame
reveal_type(frame.sort_index(inplace=True)) # None
reveal_type(frame.sort_index(inplace=False)) # DataFrame
reveal_type(frame.sort_index(inplace=bool_())) # DataFrame | None

series = pd.Series()
reveal_type(series.sort_index()) # Series
reveal_type(series.sort_index(inplace=True)) # None
reveal_type(series.sort_index(inplace=False)) # Series
reveal_type(series.sort_index(inplace=bool_())) # Series | None
```
can you merge master green, except for mypy error also present on main thanks @twoertwein  (and others!)