BUG: Series.__setitem__[listlike_of_integers] with IntervalIndex
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Changes make sense.  Just needs a `whatsnew` I think. yep if you can add a whatsnew updated with whatsnew updated + green (finally) Thanks @jbrockmendel 