BUG: Categorical scatter plot has KeyError #16199
Appropriately handles categorical data for dataframe scatter plots which
currently raises KeyError for categorical data

 - [x] closes #16199
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [x] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16208?src=pr&el=h1) Report
> Merging [#16208](https://codecov.io/gh/pandas-dev/pandas/pull/16208?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/18a428da81f7fe0f41f8ec78f03a385b731bb6a3?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `50%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16208/graphs/tree.svg?width=650&height=150&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/16208?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16208      +/-   ##
==========================================
- Coverage   90.93%   90.93%   -0.01%     
==========================================
  Files         161      161              
  Lines       49269    49273       +4     
==========================================
+ Hits        44802    44804       +2     
- Misses       4467     4469       +2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.68% <50%> (-0.01%)` | :arrow_down: |
| #single | `40.22% <0%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16208?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_core.py](https://codecov.io/gh/pandas-dev/pandas/pull/16208?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb3JlLnB5) | `81.83% <50%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16208?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/16208?src=pr&el=footer). Last update [18a428d...035e00d](https://codecov.io/gh/pandas-dev/pandas/pull/16208?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 I added the whatsnew for 0.20.1, pls rebase and add a note in bug fixes. can you rebase and update?

move release note to 0.20.3 @jreback done lgtm. @jorisvandenbossche @TomAugspurger  Thanks!