BUG/TST: assure conversions of datetimelikes for object, numeric dtypes
closes #19223
closes #12425 
This is fixed as well (and present in <= 0.22.0), I thought we had an issue about this
```
In [1]: pd.DataFrame({'A': [1,2,3]}).astype('m8[s]').dtypes
Out[1]: 
A    timedelta64[s]
dtype: object

``` # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19224?src=pr&el=h1) Report
> Merging [#19224](https://codecov.io/gh/pandas-dev/pandas/pull/19224?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/08a3d3afca2ebbb8e189c173550ec90b4a27b7d9?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `88.88%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19224/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/19224?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19224      +/-   ##
==========================================
- Coverage   91.53%   91.52%   -0.01%     
==========================================
  Files         147      147              
  Lines       48797    48797              
==========================================
- Hits        44664    44661       -3     
- Misses       4133     4136       +3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.89% <88.88%> (-0.01%)` | :arrow_down: |
| #single | `41.6% <55.55%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19224?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/internals.py](https://codecov.io/gh/pandas-dev/pandas/pull/19224/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzLnB5) | `94.5% <100%> (ø)` | :arrow_up: |
| [pandas/core/dtypes/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/19224/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbW1vbi5weQ==) | `94.58% <100%> (+0.04%)` | :arrow_up: |
| [pandas/core/dtypes/cast.py](https://codecov.io/gh/pandas-dev/pandas/pull/19224/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2Nhc3QucHk=) | `88.35% <75%> (-0.31%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/19224/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `84.2% <0%> (-0.22%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19224?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/19224?src=pr&el=footer). Last update [08a3d3a...b953695](https://codecov.io/gh/pandas-dev/pandas/pull/19224?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
