TYP: fix typing errors for mypy==0.790, bump mypy version
- [x] closes  #37104
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`


cc @simonjayhawkins  Hello @fangchenli! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:







There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2021-01-10 03:12:54 UTC looks ok to me, merge master just to confirm. Thanks @fangchenli  > merge master just to confirm.

np-dev failures seen on other PRs/master