DEPR. Change RangeIndex._start/_stop/_step to FutureWarning
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

lgtm. can you rebase. (also make sure to add to the deprecation issue tracker) lgtm pending rebase Rebased and green Thanks @mroeschke 