BUG: Avoid splitting string with list() (#20592)
 Solves #20592 and I doubt that there are any cases when a key string should be split by `list()`
This needs tests # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20615?src=pr&el=h1) Report
> Merging [#20615](https://codecov.io/gh/pandas-dev/pandas/pull/20615?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6d610a4d9393c0d0335267dc3252ccabb9e51e43?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20615/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/20615?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20615      +/-   ##
==========================================
+ Coverage   91.82%   91.84%   +0.02%     
==========================================
  Files         153      153              
  Lines       49256    49258       +2     
==========================================
+ Hits        45227    45243      +16     
+ Misses       4029     4015      -14
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.23% <100%> (+0.02%)` | :arrow_up: |
| #single | `41.9% <0%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20615?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/20615/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `93.91% <100%> (+0.01%)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/20615/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `84.73% <0%> (+0.2%)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/20615/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `66.81% <0%> (+1.73%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20615?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20615?src=pr&el=footer). Last update [6d610a4...9535274](https://codecov.io/gh/pandas-dev/pandas/pull/20615?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 And a release note: http://pandas.pydata.org/pandas-docs/stable/contributing.html#documenting-your-code closing as stale, though would certainly take a PR here