added option keep=False to nlargests/nsmallest
- [x] closes #16818
- [x] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

I made a minor change to `nlargest/nsmallest` to keep all the values of the last `n` when `keep=False`