BUG: fix failing DataFrame.loc when indexing with an IntervalIndex
- [x] closes #19977
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22576?src=pr&el=h1) Report
> Merging [#22576](https://codecov.io/gh/pandas-dev/pandas/pull/22576?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/4612312783e88d0dcd9cadfaa07077b0b382c26a?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/22576/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22576?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #22576      +/-   ##
==========================================
- Coverage   92.04%   92.04%   -0.01%     
==========================================
  Files         169      169              
  Lines       50782    50787       +5     
==========================================
+ Hits        46744    46745       +1     
- Misses       4038     4042       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.45% <100%> (-0.01%)` | :arrow_down: |
| #single | `42.29% <0%> (+0.02%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22576?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexing.py](https://codecov.io/gh/pandas-dev/pandas/pull/22576/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhpbmcucHk=) | `93.79% <100%> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/22576/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `85.75% <0%> (-0.21%)` | :arrow_down: |
| [pandas/core/ops.py](https://codecov.io/gh/pandas-dev/pandas/pull/22576/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvb3BzLnB5) | `96.76% <0%> (-0.15%)` | :arrow_down: |
| [pandas/core/resample.py](https://codecov.io/gh/pandas-dev/pandas/pull/22576/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzYW1wbGUucHk=) | `96.13% <0%> (-0.15%)` | :arrow_down: |
| [pandas/core/strings.py](https://codecov.io/gh/pandas-dev/pandas/pull/22576/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3RyaW5ncy5weQ==) | `98.63% <0%> (ø)` | :arrow_up: |
| [pandas/core/indexes/multi.py](https://codecov.io/gh/pandas-dev/pandas/pull/22576/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9tdWx0aS5weQ==) | `95.41% <0%> (ø)` | :arrow_up: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/22576/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `96.44% <0%> (ø)` | :arrow_up: |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/22576/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.45% <0%> (ø)` | :arrow_up: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/22576/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.2% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22576?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/22576?src=pr&el=footer). Last update [4612312...0888647](https://codecov.io/gh/pandas-dev/pandas/pull/22576?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 can you rebase on master Hello @sideeye! Thanks for updating the PR.

 - There are no PEP8 issues in the file [`pandas/core/indexing.py`](https://github.com/sideeye/pandas/blob/d6b156021d12ab4a4eb2eb458c9179b51140b345/pandas/core/indexing.py) !

 - There are no PEP8 issues in the file [`pandas/tests/frame/test_indexing.py`](https://github.com/sideeye/pandas/blob/d6b156021d12ab4a4eb2eb458c9179b51140b345/pandas/tests/frame/test_indexing.py) !

 I'm in doubt a little bit if i did it (rebase) the right way. @sideeye : I think you did the rebase correctly, but it looks like there are test failures because of your change...not sure if they're actually related though... thanks @sideeye 