BUG: unexpected assign by a single-element list (GH19474)
- [x] closes #19474
- [x] tests added / passed (some failed independently of my changes)
- [x] passes `git diff master --name-only -- "*.py" | flake8`
- [x] whatsnew entry

I thought it is a straightforward way to evaluate all of `df.loc[0, ['A']] = ['X']`, `df.loc[0, ['A', 'B']] = ['X', 'Y']`, ...(more-columns case) the same way in `else` block at line 590 of pandas/core/indexing.py.

Please let me know bad or ungrammatical points.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20732?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@23bc217`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20732/graphs/tree.svg?height=150&src=pr&token=eZ4WkYLtcO&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/20732?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #20732   +/-   ##
=========================================
  Coverage          ?   91.84%           
=========================================
  Files             ?      153           
  Lines             ?    49295           
  Branches          ?        0           
=========================================
  Hits              ?    45276           
  Misses            ?     4019           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.23% <100%> (?)` | |
| #single | `41.89% <0%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20732?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexing.py](https://codecov.io/gh/pandas-dev/pandas/pull/20732/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhpbmcucHk=) | `93.08% <100%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20732?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/20732?src=pr&el=footer). Last update [23bc217...bfa2f67](https://codecov.io/gh/pandas-dev/pandas/pull/20732?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @kittoku keep em coming!