CLN: Use C-API for datetime.date
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

LGTM pending green For my own understanding, why do we prefer to use the C API? Is this likely to improve performance? >  Is this likely to improve performance?

@TomAugspurger Correct! 

---

This is because when we use ```import``` and not ```cimport``` we are going through the python space, which is slower than pure C.

I have made POC so you could see to diffs:

![date_vs_Cdate](https://user-images.githubusercontent.com/50263213/78190573-ef6ebc80-747c-11ea-8cab-fa5fe06856fe.png)

Both are doing the exact same thing, only that the C-API ```datetime.date``` is much faster. Thanks @MomIsBestFriend 