ENH: Add public start, stop, and step attributes to RangeIndex
- [X] closes #25710
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

Comments:
- Was originally going to add the tests for this to `test_constructor` but decided to create a new test instead. I parametrized `test_constructor` to clean it up before deciding to create a separate test and kept those changes in the PR.
- Copied the `start`/`stop`/`step` docstrings from the Python docs for `range` with a small rewording to fit it on one line
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25720?src=pr&el=h1) Report
> Merging [#25720](https://codecov.io/gh/pandas-dev/pandas/pull/25720?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/10173821965fa65654b172891b2102a8426132ca?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/25720/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25720?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25720      +/-   ##
==========================================
- Coverage   91.24%   91.24%   -0.01%     
==========================================
  Files         172      172              
  Lines       52967    52973       +6     
==========================================
+ Hits        48332    48337       +5     
- Misses       4635     4636       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.82% <100%> (ø)` | :arrow_up: |
| #single | `41.75% <50%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25720?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/range.py](https://codecov.io/gh/pandas-dev/pandas/pull/25720/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9yYW5nZS5weQ==) | `97.41% <100%> (+0.04%)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25720/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `88.98% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25720?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/25720?src=pr&el=footer). Last update [1017382...70bd4d6](https://codecov.io/gh/pandas-dev/pandas/pull/25720?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/25720?src=pr&el=h1) Report
> Merging [#25720](https://codecov.io/gh/pandas-dev/pandas/pull/25720?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/10173821965fa65654b172891b2102a8426132ca?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/25720/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25720?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25720      +/-   ##
==========================================
- Coverage   91.24%   91.24%   -0.01%     
==========================================
  Files         172      172              
  Lines       52967    52973       +6     
==========================================
+ Hits        48332    48337       +5     
- Misses       4635     4636       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.82% <100%> (ø)` | :arrow_up: |
| #single | `41.75% <50%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25720?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/range.py](https://codecov.io/gh/pandas-dev/pandas/pull/25720/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9yYW5nZS5weQ==) | `97.41% <100%> (+0.04%)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25720/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `88.98% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25720?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/25720?src=pr&el=footer). Last update [1017382...70bd4d6](https://codecov.io/gh/pandas-dev/pandas/pull/25720?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @jschendel very nice