REF: DatetimeIndex constructor fixups
Small non-central pieces broken off from #23675.
Hello @jbrockmendel! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/arrays/datetimes.py`](https://github.com/jbrockmendel/pandas/blob/2e7e5d9b209006a3c99ea688864644b8ddee9f7a/pandas/core/arrays/datetimes.py) !

 - There are no PEP8 issues in the file [`pandas/core/indexes/datetimes.py`](https://github.com/jbrockmendel/pandas/blob/2e7e5d9b209006a3c99ea688864644b8ddee9f7a/pandas/core/indexes/datetimes.py) !

 - There are no PEP8 issues in the file [`pandas/tests/scalar/timedelta/test_timedelta.py`](https://github.com/jbrockmendel/pandas/blob/2e7e5d9b209006a3c99ea688864644b8ddee9f7a/pandas/tests/scalar/timedelta/test_timedelta.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23916?src=pr&el=h1) Report
> Merging [#23916](https://codecov.io/gh/pandas-dev/pandas/pull/23916?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/1f02bf240c3d0d3da338af868d056bfc169b28c2?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23916/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23916?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23916   +/-   ##
=======================================
  Coverage   92.29%   92.29%           
=======================================
  Files         161      161           
  Lines       51500    51500           
=======================================
  Hits        47533    47533           
  Misses       3967     3967
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.69% <100%> (ø)` | :arrow_up: |
| #single | `42.42% <100%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23916?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/23916/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `96.2% <100%> (ø)` | :arrow_up: |
| [pandas/core/arrays/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/23916/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lcy5weQ==) | `98.51% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23916?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/23916?src=pr&el=footer). Last update [1f02bf2...2e7e5d9](https://codecov.io/gh/pandas-dev/pandas/pull/23916?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @jbrockmendel 