TYP: Extend typing to allow passing dates to to_datetime
Updates the typing to reflect what is already possible.

Otherwise, Pycharm is pretty unhappy when a date is pased.

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

needs to merge master and update This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this. closing as stale. if you want to continue, pls ping and can re-open.