ENH: Add sort parameter to RangeIndex.union (#24471)
- [ ] progress towards #24471
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

This is WIP for adding a ``sort`` parameter to ``RangeIndex.union`` that behaves in a similar way to the other index types.

``sort=None`` is the default to make it consistent with the union method in the base class. When ``sort=None`` a monotonically increasing ``RangeIndex`` will be returned if possible and a sorted ``Int64Index`` if not.

The way I have implemented ``sort=False`` is that it returns an ``Int64Index`` in all cases. I have been trying to think of cases where it would make sense to still return a ``RangeIndex`` when ``sort=False``. For example, there might be a case where if we had two ``RangeIndex``s and both had the same step and the second ``RangeIndex`` overlapped with the first we would want to return a ``RangeIndex`` here even if we had ``sort=False``. But would it be better just to always return an ``Int64Index`` when ``sort=False`` as I have done here to make the return type consistent and because this particular case seems quite rare? 

```
# sort=False returns an Int64Index even though we might be able to return a RangeIndex as below
In [1]: RangeIndex(0, 10, 2).union(RangeIndex(10, 12, 2), sort=False)
Out[1]: Int64Index([0, 2, 4, 6, 8, 10], dtype='int64')

In [1]: RangeIndex(0, 10, 2).union(RangeIndex(10, 12, 2), sort=None)
Out[1]: RangeIndex(start=0, stop=12, step=2)
```
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25788?src=pr&el=h1) Report
> Merging [#25788](https://codecov.io/gh/pandas-dev/pandas/pull/25788?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/8e54e556a6083c3ef26f246db36fb0ac556e846f?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25788/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25788?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25788      +/-   ##
==========================================
+ Coverage   91.26%   91.26%   +<.01%     
==========================================
  Files         172      172              
  Lines       52965    52965              
==========================================
+ Hits        48337    48338       +1     
+ Misses       4628     4627       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.82% <100%> (ø)` | :arrow_up: |
| #single | `41.74% <60%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25788?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/25788/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.57% <100%> (ø)` | :arrow_up: |
| [pandas/core/indexes/range.py](https://codecov.io/gh/pandas-dev/pandas/pull/25788/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9yYW5nZS5weQ==) | `97.41% <100%> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25788/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `89.44% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25788?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/25788?src=pr&el=footer). Last update [8e54e55...fadd52e](https://codecov.io/gh/pandas-dev/pandas/pull/25788?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/25788?src=pr&el=h1) Report
> Merging [#25788](https://codecov.io/gh/pandas-dev/pandas/pull/25788?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/8e54e556a6083c3ef26f246db36fb0ac556e846f?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25788/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25788?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25788      +/-   ##
==========================================
+ Coverage   91.26%   91.26%   +<.01%     
==========================================
  Files         172      172              
  Lines       52965    52965              
==========================================
+ Hits        48337    48338       +1     
+ Misses       4628     4627       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.82% <100%> (ø)` | :arrow_up: |
| #single | `41.74% <60%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25788?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/25788/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.57% <100%> (ø)` | :arrow_up: |
| [pandas/core/indexes/range.py](https://codecov.io/gh/pandas-dev/pandas/pull/25788/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9yYW5nZS5weQ==) | `97.41% <100%> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25788/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `89.44% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25788?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/25788?src=pr&el=footer). Last update [8e54e55...fadd52e](https://codecov.io/gh/pandas-dev/pandas/pull/25788?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/25788?src=pr&el=h1) Report
> Merging [#25788](https://codecov.io/gh/pandas-dev/pandas/pull/25788?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/923ac2bdee409e4fa8c47414b07f52e036bb21bc?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/25788/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25788?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25788      +/-   ##
==========================================
- Coverage   91.48%   91.48%   -0.01%     
==========================================
  Files         175      175              
  Lines       52885    52885              
==========================================
- Hits        48381    48380       -1     
- Misses       4504     4505       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.04% <100%> (ø)` | :arrow_up: |
| #single | `41.82% <60%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25788?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/25788/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.57% <100%> (ø)` | :arrow_up: |
| [pandas/core/indexes/range.py](https://codecov.io/gh/pandas-dev/pandas/pull/25788/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9yYW5nZS5weQ==) | `97.41% <100%> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25788/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `89.74% <0%> (-0.11%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25788?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/25788?src=pr&el=footer). Last update [923ac2b...c32ca8b](https://codecov.io/gh/pandas-dev/pandas/pull/25788?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Hello @reidy-p! 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 2019-03-26 16:02:11 UTC thanks @reidy-p 