BUG: Fix wrong khash method definition
- [x] closes #20722
- [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/20966?src=pr&el=h1) Report
> Merging [#20966](https://codecov.io/gh/pandas-dev/pandas/pull/20966?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/fc50bde73284eeb808a485c0d22c30a0cc6becaf?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20966/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/20966?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #20966   +/-   ##
=======================================
  Coverage   91.81%   91.81%           
=======================================
  Files         153      153           
  Lines       49487    49487           
=======================================
  Hits        45437    45437           
  Misses       4050     4050
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.21% <ø> (ø)` | :arrow_up: |
| #single | `41.85% <ø> (ø)` | :arrow_up: |


------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20966?src=pr&el=footer). Last update [fc50bde...375c656](https://codecov.io/gh/pandas-dev/pandas/pull/20966?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks @jreback, fixed! lgtm. ping on green. I pushed some cleaning of the test. thanks @Licht-T 