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.

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26581?src=pr&el=h1) Report
> Merging [#26581](https://codecov.io/gh/pandas-dev/pandas/pull/26581?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/4c54dd298692783f417cbaa57d5fc1c0dc1f7c72?src=pr&el=desc) will **decrease** coverage by `50.17%`.
> The diff coverage is `40%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26581/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26581?src=pr&el=tree)

```diff
@@             Coverage Diff             @@
##           master   #26581       +/-   ##
===========================================
- Coverage   91.84%   41.67%   -50.18%     
===========================================
  Files         174      174               
  Lines       50644    50625       -19     
===========================================
- Hits        46516    21099    -25417     
- Misses       4128    29526    +25398
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `?` | |
| #single | `41.67% <40%> (-0.13%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26581?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/packers.py](https://codecov.io/gh/pandas-dev/pandas/pull/26581/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhY2tlcnMucHk=) | `14.53% <ø> (-73.55%)` | :arrow_down: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/26581/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `45.31% <ø> (-48.31%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26581/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `34.99% <ø> (-62.13%)` | :arrow_down: |
| [pandas/core/indexes/range.py](https://codecov.io/gh/pandas-dev/pandas/pull/26581/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9yYW5nZS5weQ==) | `49.84% <35.38%> (-48.13%)` | :arrow_down: |
| [pandas/core/dtypes/concat.py](https://codecov.io/gh/pandas-dev/pandas/pull/26581/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbmNhdC5weQ==) | `52.19% <70%> (-44.38%)` | :arrow_down: |
| [pandas/io/formats/latex.py](https://codecov.io/gh/pandas-dev/pandas/pull/26581/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvbGF0ZXgucHk=) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/io/sas/sas\_constants.py](https://codecov.io/gh/pandas-dev/pandas/pull/26581/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3Nhcy9zYXNfY29uc3RhbnRzLnB5) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/core/groupby/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/26581/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9jYXRlZ29yaWNhbC5weQ==) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/tseries/plotting.py](https://codecov.io/gh/pandas-dev/pandas/pull/26581/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvcGxvdHRpbmcucHk=) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/tseries/converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/26581/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvY29udmVydGVyLnB5) | `0% <0%> (-100%)` | :arrow_down: |
| ... and [133 more](https://codecov.io/gh/pandas-dev/pandas/pull/26581/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26581?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26581?src=pr&el=footer). Last update [4c54dd2...d947faa](https://codecov.io/gh/pandas-dev/pandas/pull/26581?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26581?src=pr&el=h1) Report
> Merging [#26581](https://codecov.io/gh/pandas-dev/pandas/pull/26581?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/454b8c5cdcea0cbba981d607293b990cc704f3a1?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26581/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26581?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26581      +/-   ##
==========================================
- Coverage   91.87%   91.87%   -0.01%     
==========================================
  Files         174      174              
  Lines       50692    50672      -20     
==========================================
- Hits        46575    46554      -21     
- Misses       4117     4118       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.41% <100%> (ø)` | :arrow_up: |
| #single | `41.78% <42.99%> (-0.1%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26581?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/packers.py](https://codecov.io/gh/pandas-dev/pandas/pull/26581/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhY2tlcnMucHk=) | `88.08% <ø> (ø)` | :arrow_up: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/26581/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `93.61% <ø> (ø)` | :arrow_up: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26581/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97% <ø> (-0.12%)` | :arrow_down: |
| [pandas/core/dtypes/concat.py](https://codecov.io/gh/pandas-dev/pandas/pull/26581/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbmNhdC5weQ==) | `96.58% <100%> (+0.01%)` | :arrow_up: |
| [pandas/core/dtypes/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/26581/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbW1vbi5weQ==) | `96% <100%> (+0.14%)` | :arrow_up: |
| [pandas/core/indexes/range.py](https://codecov.io/gh/pandas-dev/pandas/pull/26581/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9yYW5nZS5weQ==) | `98.47% <100%> (+0.42%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26581/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/26581/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.91% <0%> (+0.1%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26581?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26581?src=pr&el=footer). Last update [454b8c5...228b560](https://codecov.io/gh/pandas-dev/pandas/pull/26581?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 also a note in the whatsnew for the deprecation Hello @topper-123! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:















There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2019-06-04 09:18:53 UTC lgtm. @TomAugspurger @jorisvandenbossche any objections? > Do we even need to deprecate these or can we just remove? They technically were private before so don't want to set a precedent that have to deprecate internal code going forward as well if we don't have to

@WillAyd see comments above, they are using in ``pyarrow`` & ``fastparquet`` use them. So best to deprecate.
 Whoops sorry - thanks for clarifying What's the plan on warning? Should we throw a (not visible) DeprecationWarning for now, and promote that to a FutureWarning? Or just add a FutureWarning after pyarrow / fastparquet can update? My plan is to just post an issue on the issue lists for ``pyarrow`` & ``fastparquet`` with a notice that the attributes will be removed in 1.0 (unless they want us to hold on longer). I think that is sufficient as these are not publicly advertised.

EDIT: My cercern for adding a code message was that it would be visible and therefore annoying for users of those packages. I could add a DeprecationWarning, if that one is more quiet than the usual FutureWarning. ok let’s do a DeprecationWarning now
and then post issues should be good I actually see now that DeprecationWarnings are also displayed by default. I thought they were only shown when starting Python with ``-Wall`` or similar. Am I missing something?

Uploading anyway now I've done it and there's a demand for the warnings. > I actually see now that DeprecationWarnings are also displayed by default

Only if you directly use the deprecated method or attribute, but they should not show up if you eg call a function that uses them (which should be the case for libraries such as pyarrow / fastparquet).

 Personally, I am -1 on deprecating them (pyarrow/fastparquet only just started to use them, we literally said them it was fine, it's more annoying for them to deal with an attribute name change than for us to just keep it a couple of releases more). But if we're using a deprecation instead of future warning, I won't object if the majority is fine with deprecating. > Personally, I am -1 on deprecating them (pyarrow/fastparquet only just started to use them, we literally said them it was fine, it's more annoying for them to deal with an attribute name change than for us to just keep it a couple of releases more). But if we're using a deprecation instead of future warning, I won't object if the majority is fine with deprecating.

right this is the purpose of the DeprecationWarning (rather than FutureWarning) as it won't show-up too much; but the deprecation should happen. @topper-123 can you file an issue on fastparquet and a JIRA on pyarrow. No problem. I'd like to have this merged before I write the issues, so they can see the issue by just pulling pandas master. Just added a small clean-up, so ``__new__`` won't have a custom & internal ``ensure_int``, but uses the same ``ensure_python_int`` as proposed in #26617. This simplifies the code a bit further.

If this gets accepted before #26617, I'll rebase that and add the requested type hints.
 thanks @topper-123 