BUG: Fix IntervalIndex.get_loc/get_indexer for IntervalIndex of length one
- [X] closes #17284
- [X] closes #20921
- [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/20946?src=pr&el=h1) Report
> Merging [#20946](https://codecov.io/gh/pandas-dev/pandas/pull/20946?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/28dbae9f306ade549eb1edd5484b3e1da758bcdb?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20946/graphs/tree.svg?height=150&src=pr&token=eZ4WkYLtcO&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/20946?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20946      +/-   ##
==========================================
- Coverage   91.81%   91.81%   -0.01%     
==========================================
  Files         153      153              
  Lines       49479    49478       -1     
==========================================
- Hits        45428    45427       -1     
  Misses       4051     4051
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.2% <ø> (-0.01%)` | :arrow_down: |
| #single | `41.85% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20946?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/interval.py](https://codecov.io/gh/pandas-dev/pandas/pull/20946/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9pbnRlcnZhbC5weQ==) | `93.08% <ø> (ø)` | :arrow_up: |
| [pandas/core/strings.py](https://codecov.io/gh/pandas-dev/pandas/pull/20946/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3RyaW5ncy5weQ==) | `98.62% <0%> (-0.01%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20946?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/20946?src=pr&el=footer). Last update [28dbae9...3c82ef3](https://codecov.io/gh/pandas-dev/pandas/pull/20946?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @jschendel. always nice to have a great patch from you!