STYLE: Run Isort on tests-> util, series, arrays
- [x] partial #23334
- [x] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
Run `isort` on some leftover files from `tests/util`, `tests/arrays` and `tests/series`. 

Hello @thoo! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/tests/arrays/categorical/test_missing.py`](https://github.com/thoo/pandas/blob/295febf8affaddf0dbde21cd69b62396ef75a20b/pandas/tests/arrays/categorical/test_missing.py) !

 - There are no PEP8 issues in the file [`pandas/tests/arrays/categorical/test_sorting.py`](https://github.com/thoo/pandas/blob/295febf8affaddf0dbde21cd69b62396ef75a20b/pandas/tests/arrays/categorical/test_sorting.py) !

 - There are no PEP8 issues in the file [`pandas/tests/arrays/test_datetimelike.py`](https://github.com/thoo/pandas/blob/295febf8affaddf0dbde21cd69b62396ef75a20b/pandas/tests/arrays/test_datetimelike.py) !

 - There are no PEP8 issues in the file [`pandas/tests/arrays/test_integer.py`](https://github.com/thoo/pandas/blob/295febf8affaddf0dbde21cd69b62396ef75a20b/pandas/tests/arrays/test_integer.py) !

 - There are no PEP8 issues in the file [`pandas/tests/series/test_duplicates.py`](https://github.com/thoo/pandas/blob/295febf8affaddf0dbde21cd69b62396ef75a20b/pandas/tests/series/test_duplicates.py) !

 - There are no PEP8 issues in the file [`pandas/tests/util/test_hashing.py`](https://github.com/thoo/pandas/blob/295febf8affaddf0dbde21cd69b62396ef75a20b/pandas/tests/util/test_hashing.py) !

 - There are no PEP8 issues in the file [`pandas/tests/util/test_testing.py`](https://github.com/thoo/pandas/blob/295febf8affaddf0dbde21cd69b62396ef75a20b/pandas/tests/util/test_testing.py) !

 - There are no PEP8 issues in the file [`pandas/tests/util/test_util.py`](https://github.com/thoo/pandas/blob/295febf8affaddf0dbde21cd69b62396ef75a20b/pandas/tests/util/test_util.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23501?src=pr&el=h1) Report
> Merging [#23501](https://codecov.io/gh/pandas-dev/pandas/pull/23501?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/24ab22f75aae1c9b9491c26e6f753814b7d6fd8e?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23501/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23501?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23501   +/-   ##
=======================================
  Coverage   92.23%   92.23%           
=======================================
  Files         161      161           
  Lines       51197    51197           
=======================================
  Hits        47220    47220           
  Misses       3977     3977
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.61% <ø> (ø)` | :arrow_up: |
| #single | `42.27% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23501?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/23501?src=pr&el=footer). Last update [24ab22f...295febf](https://codecov.io/gh/pandas-dev/pandas/pull/23501?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks! thanks! @thoo 