TYP: fix ReadPickleBuffer
This makes ReadPickleBuffer compatible with `tempfile.NamedTemporaryFile`:

- `tempfile.NamedTemporaryFile`'s `read` does not accept `None` (luckily we call none of the Buffer classes with `None`)
- `readline` returned a `TypeVar` instead of a `bytes`


The following works now:

```py
# pyright: strict
import tempfile
from pandas._typing import ReadPickleBuffer

def read(x: ReadPickleBuffer) -> None:
    ...

with tempfile.NamedTemporaryFile(delete=False) as file:
    read(file)
```
Could you merge in main once more? Looked good but not sure if the pyright bump will catch anything else since the last merge here > Could you merge in main once more? Looked good but not sure if the pyright bump will catch anything else since the last merge here

Rebased (and bumped pyright again) Thanks @twoertwein 