CLN: Added annotations.
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

I'm really confused about how it failed.  Don't think the test failures are related to this change - I see the same on the latest PR that was merged -
https://github.com/pandas-dev/pandas/runs/295247988 I'm now even more confused, about the fact that it passed. > I'm now even more confused, about the fact that it passed.

The test is unreliable ive raised an issue here: https://github.com/pandas-dev/pandas/issues/29514 Thanks @MomIsBestFriend 