BUG: nunique not ignoring both None and np.nan
- [x] closes #37566
- [x] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Hello @GYHHAHA! 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 2020-11-10 01:20:07 UTC looks good, just @phofl comment i think is left, ping on green. cc @jreback  thanks @GYHHAHA very nice