fix: scalar timestamp assignment (#19843)
- [x] closes #19843
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

what should I do for the `whatsnew`? Perhaps something like:

~~~
Fixed `DataFrame` assignment with a timezone-aware object (:issue:`19843`)
~~~ # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19973?src=pr&el=h1) Report
> Merging [#19973](https://codecov.io/gh/pandas-dev/pandas/pull/19973?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/cf1462767e61466746c9bba5e71ebd0f1e87d5e3?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19973/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/19973?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19973      +/-   ##
==========================================
+ Coverage   92.06%   92.06%   +<.01%     
==========================================
  Files         169      169              
  Lines       50681    50690       +9     
==========================================
+ Hits        46659    46668       +9     
  Misses       4022     4022
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.47% <100%> (ø)` | :arrow_up: |
| #single | `42.32% <100%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19973?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/19973/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.26% <100%> (ø)` | :arrow_up: |
| [pandas/io/parsers.py](https://codecov.io/gh/pandas-dev/pandas/pull/19973/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnNlcnMucHk=) | `95.48% <0%> (+0.02%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19973?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/19973?src=pr&el=footer). Last update [cf14627...1b9649c](https://codecov.io/gh/pandas-dev/pandas/pull/19973?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 can you rebase and update can you rebase & move the note to 0.24.0 fixed up, but going to do in ~0.23.4.~ 0.24.0