CLN import from pandas.core.arrays when possible
- [ ] closes #xxxx
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

`pandas.core.arrays.__init__.py` imports some classes by default, so other classes can import directly from `pandas.core.arrays` instead of accessing the specific files
Hello @JustinZhengBC! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/arrays/datetimes.py`](https://github.com/JustinZhengBC/pandas/blob/9ccaf51b1bf50d1b92076cc097504fd958a9049f/pandas/core/arrays/datetimes.py) !

 - There are no PEP8 issues in the file [`pandas/core/arrays/period.py`](https://github.com/JustinZhengBC/pandas/blob/9ccaf51b1bf50d1b92076cc097504fd958a9049f/pandas/core/arrays/period.py) !

 - There are no PEP8 issues in the file [`pandas/core/groupby/generic.py`](https://github.com/JustinZhengBC/pandas/blob/9ccaf51b1bf50d1b92076cc097504fd958a9049f/pandas/core/groupby/generic.py) !

 - There are no PEP8 issues in the file [`pandas/tests/arrays/test_datetimelike.py`](https://github.com/JustinZhengBC/pandas/blob/9ccaf51b1bf50d1b92076cc097504fd958a9049f/pandas/tests/arrays/test_datetimelike.py) !

 lgtm. we can merge on green. ( # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23193?src=pr&el=h1) Report
> Merging [#23193](https://codecov.io/gh/pandas-dev/pandas/pull/23193?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/cd863d390b2f20cb144c928cf3c9f154ab7ccfe3?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/23193/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23193?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23193   +/-   ##
=======================================
  Coverage   92.19%   92.19%           
=======================================
  Files         169      169           
  Lines       50964    50964           
=======================================
  Hits        46985    46985           
  Misses       3979     3979
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.61% <100%> (ø)` | :arrow_up: |
| #single | `42.29% <25%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23193?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/arrays/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/23193/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3BlcmlvZC5weQ==) | `95.56% <100%> (ø)` | :arrow_up: |
| [pandas/core/groupby/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/23193/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9nZW5lcmljLnB5) | `87.06% <100%> (ø)` | :arrow_up: |
| [pandas/core/arrays/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/23193/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lcy5weQ==) | `97.38% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23193?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/23193?src=pr&el=footer). Last update [cd863d3...9ccaf51](https://codecov.io/gh/pandas-dev/pandas/pull/23193?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Green enough :)

Thanks @JustinZhengBC.