ENH: Allow multi values for index and columns in df.pivot
- [x] closes #21425
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Hello @charlesdong1991! 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 2020-02-02 19:24:06 UTC would be nice if someone could take a look here, i just rebased ^^ just in case if you'd like to take a look too @WillAyd thanks! ^^ HI, @jreback  and @WillAyd thanks for both of yours reviews, appreciate a lot!!

One thing to mention: i'd like to have signature added as a follow-up to make this clearer, since we have PRs addressing TYPING issues only if you agree. Otherwise, I think I have addressed both of your reviews in the change ^^

Feel free for further reviews! Thanks!  just updated with latest master gentle ping @WillAyd for feedback? ^^ gentle ping @jreback ^^ sorry for pushing for a review, I know you are so busy with so many PRs. This PR has been rebased several times, so some feedbacks would be appreciated @jreback 👍  thanks @charlesdong1991 
very nice