CLN: Added static types
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

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I'll revert any change related to styling, I applied "black" formatting in some places, just so I could think straight.
lgtm @jreback any outstanding comments? thanks @MomIsBestFriend 