core: fix DatetimeBlock operated with timedelta
- [X] closes #22005
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22007?src=pr&el=h1) Report
> Merging [#22007](https://codecov.io/gh/pandas-dev/pandas/pull/22007?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/2c7c79779f1ef353d02446504d3b5f5c5cc68759?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/22007/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/22007?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #22007      +/-   ##
==========================================
+ Coverage   92.02%   92.02%   +<.01%     
==========================================
  Files         170      170              
  Lines       50710    50715       +5     
==========================================
+ Hits        46664    46669       +5     
  Misses       4046     4046
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.43% <100%> (ø)` | :arrow_up: |
| #single | `42.3% <0%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22007?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/internals/blocks.py](https://codecov.io/gh/pandas-dev/pandas/pull/22007/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL2Jsb2Nrcy5weQ==) | `94.47% <100%> (+0.01%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22007?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/22007?src=pr&el=footer). Last update [2c7c797...518b3aa](https://codecov.io/gh/pandas-dev/pandas/pull/22007?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 cc @jbrockmendel  Pending requested fleshing out in tests (and rebase), this looks like a fine stop-gap solution until we rip out Block-based arithmetic altogether. The issue #22005 this was written for was closed by #22163.  @holymonson are there other bugs this fixes? >The issue #22005 this was written for was closed by #22163. @holymonson are there other bugs this fixes?

Nope, I will close this.