BUG: IntervalArray.__eq__ not deferring to Series
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

This changes the result type of `interval_array == series`? We'll need a whatsnew for that (1.2).

Looks good otherwise. conflict resolved, merge on green-ish