TYP: mypy complaint isocalendar does not match supertype
### 

- [X] I have checked that this issue has not already been reported.

- [X] I have confirmed this bug exists on the [latest version](https://pandas.pydata.org/docs/whatsnew/index.html) of pandas.

- [X] I have confirmed this bug exists on the master branch of pandas.


### Reproducible Example

```python
When I run mypy locally (this has happened for a while and I've been ignoring it) I get


pandas/_libs/tslibs/timestamps.pyi:160: error: Return type "Tuple[int, int, int]" of "isocalendar" incompatible with return type "_IsoCalendarDate" in supertype "datetime"  [override]
pandas/_libs/tslibs/timestamps.pyi:160: error: Return type "Tuple[int, int, int]" of "isocalendar" incompatible with return type "_IsoCalendarDate" in supertype "date"  [override]
```
```


### Issue Description

N/A

### Expected Behavior

N/A

### Installed Versions

py39, master