DOC: Correct import location for extension classes
Minor correction of import location for ``ExtensionArray`` and ``ExtensionScalarOpsMixin``.

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24456?src=pr&el=h1) Report
> Merging [#24456](https://codecov.io/gh/pandas-dev/pandas/pull/24456?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/3086e0a170934d87b01fa910f371002a55fd07b9?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/24456/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24456?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24456   +/-   ##
=======================================
  Coverage   92.31%   92.31%           
=======================================
  Files         163      163           
  Lines       51987    51987           
=======================================
  Hits        47990    47990           
  Misses       3997     3997
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.72% <ø> (ø)` | :arrow_up: |
| #single | `42.99% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24456?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/24456?src=pr&el=footer). Last update [3086e0a...73253d6](https://codecov.io/gh/pandas-dev/pandas/pull/24456?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/24456?src=pr&el=h1) Report
> Merging [#24456](https://codecov.io/gh/pandas-dev/pandas/pull/24456?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/3086e0a170934d87b01fa910f371002a55fd07b9?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/24456/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24456?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24456   +/-   ##
=======================================
  Coverage   92.31%   92.31%           
=======================================
  Files         163      163           
  Lines       51987    51987           
=======================================
  Hits        47990    47990           
  Misses       3997     3997
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.72% <ø> (ø)` | :arrow_up: |
| #single | `42.99% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24456?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/24456?src=pr&el=footer). Last update [3086e0a...73253d6](https://codecov.io/gh/pandas-dev/pandas/pull/24456?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!