BUG: Fix regression for RGB(A) color arguments
 - [ ] closes #16695
 - [x] test added / passed
 - [x] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [x] whatsnew entry

Fixes a regression in the plot API in which RGB and RGBA tuples in the `color` kwarg caused an error when passed to matplotlib.  The root cause of this error is the fact that anything list-like passed with the `color` kwarg is treated as a set of colors to be mapped to the series being plotted.

This PR specifically fixes issues where a series is being plotted using an RGB(A) tuple to specify color.

This fixes only the regression between previous versions and 0.20.2.  There are still API inconsistencies with color specification using RGB(A) tuples versus other methods, e.g.  
```python
df = pandas.DataFrame({'a': [1, 2], 'b': [3, 4]})
df.plot(color=(1, 0, 0))  # will still fail since there are 2 columns to plot
df.plot(color='r')  # works
```


# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16701?src=pr&el=h1) Report
> Merging [#16701](https://codecov.io/gh/pandas-dev/pandas/pull/16701?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/b7e7fd3f17d4d2a2f87b9d169cf87143f04e5d33?src=pr&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16701/graphs/tree.svg?token=eZ4WkYLtcO&width=650&src=pr&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/16701?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16701      +/-   ##
==========================================
+ Coverage   90.91%   90.93%   +0.01%     
==========================================
  Files         161      161              
  Lines       49286    49288       +2     
==========================================
+ Hits        44810    44821      +11     
+ Misses       4476     4467       -9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.7% <100%> (+0.01%)` | :arrow_up: |
| #single | `40.17% <0%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16701?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_core.py](https://codecov.io/gh/pandas-dev/pandas/pull/16701?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb3JlLnB5) | `82.52% <100%> (+0.69%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16701?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/16701?src=pr&el=footer). Last update [b7e7fd3...9fa9971](https://codecov.io/gh/pandas-dev/pandas/pull/16701?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/16701?src=pr&el=h1) Report
> Merging [#16701](https://codecov.io/gh/pandas-dev/pandas/pull/16701?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/b7e7fd3f17d4d2a2f87b9d169cf87143f04e5d33?src=pr&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16701/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/16701?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16701      +/-   ##
==========================================
+ Coverage   90.91%   90.93%   +0.01%     
==========================================
  Files         161      161              
  Lines       49286    49288       +2     
==========================================
+ Hits        44810    44821      +11     
+ Misses       4476     4467       -9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.7% <100%> (+0.01%)` | :arrow_up: |
| #single | `40.17% <0%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16701?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_core.py](https://codecov.io/gh/pandas-dev/pandas/pull/16701?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb3JlLnB5) | `82.52% <100%> (+0.69%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16701?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/16701?src=pr&el=footer). Last update [b7e7fd3...d61d17a](https://codecov.io/gh/pandas-dev/pandas/pull/16701?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jorisvandenbossche thanks for the comments. Ready for another look. Thanks @kjford, sorry for breaking this earlier :)