Fix import format at pandas/tests/io/dtypes directory
- [x] partial #23334 
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Ran` isort --recursive pandas/tests/io/dtypes` and then checked imports using `isort --recursive --check-only pandas/tests/io/dtypes`
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24578?src=pr&el=h1) Report
> Merging [#24578](https://codecov.io/gh/pandas-dev/pandas/pull/24578?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/9ad1e00c5c70b4446f2af5bea9089ca3312a3eb5?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/24578/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24578?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24578   +/-   ##
=======================================
  Coverage   92.33%   92.33%           
=======================================
  Files         166      166           
  Lines       52544    52544           
=======================================
  Hits        48516    48516           
  Misses       4028     4028
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.76% <ø> (ø)` | :arrow_up: |
| #single | `43.05% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24578?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/24578?src=pr&el=footer). Last update [9ad1e00...bb61b47](https://codecov.io/gh/pandas-dev/pandas/pull/24578?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24578?src=pr&el=h1) Report
> Merging [#24578](https://codecov.io/gh/pandas-dev/pandas/pull/24578?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/9ad1e00c5c70b4446f2af5bea9089ca3312a3eb5?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/24578/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24578?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24578   +/-   ##
=======================================
  Coverage   92.33%   92.33%           
=======================================
  Files         166      166           
  Lines       52544    52544           
=======================================
  Hits        48516    48516           
  Misses       4028     4028
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.76% <ø> (ø)` | :arrow_up: |
| #single | `43.05% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24578?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/24578?src=pr&el=footer). Last update [9ad1e00...bb61b47](https://codecov.io/gh/pandas-dev/pandas/pull/24578?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks !