API: allow step!=1 slice 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

See discussion https://github.com/pandas-dev/pandas/pull/16386#discussion_r374807689

cc @jorisvandenbossche @jschendel 