use range in RangeIndex instead of _start etc.
- [x] xref #26565
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Make RangeIndex use python's ``range`` internally, rather than the three scalars ``_start``, ``_stop``  and ``_step``.

Python3's ``range`` has several nice properties, that were not available in ``xrange`` in Python2. For example it's sliceable, and has ``.index`` and ``count`` methods. These can be used in Pandas, rather than maintaining Pandas-specific code, offering cleaner code and possibly faster operations.
