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

PR includes 21 files, but I think it's acceptable and unnecessary to divide it to two separate PRs.

Hello @mwoss! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/tests/extension/arrow/bool.py`](https://github.com/mwoss/pandas/blob/f1f1fb76b903ad893065b0bd660bbf05debe472e/pandas/tests/extension/arrow/bool.py) !

 - There are no PEP8 issues in the file [`pandas/tests/extension/arrow/test_bool.py`](https://github.com/mwoss/pandas/blob/f1f1fb76b903ad893065b0bd660bbf05debe472e/pandas/tests/extension/arrow/test_bool.py) !

 - There are no PEP8 issues in the file [`pandas/tests/extension/base/dtype.py`](https://github.com/mwoss/pandas/blob/f1f1fb76b903ad893065b0bd660bbf05debe472e/pandas/tests/extension/base/dtype.py) !

 - There are no PEP8 issues in the file [`pandas/tests/extension/base/getitem.py`](https://github.com/mwoss/pandas/blob/f1f1fb76b903ad893065b0bd660bbf05debe472e/pandas/tests/extension/base/getitem.py) !

 - There are no PEP8 issues in the file [`pandas/tests/extension/base/groupby.py`](https://github.com/mwoss/pandas/blob/f1f1fb76b903ad893065b0bd660bbf05debe472e/pandas/tests/extension/base/groupby.py) !

 - There are no PEP8 issues in the file [`pandas/tests/extension/base/methods.py`](https://github.com/mwoss/pandas/blob/f1f1fb76b903ad893065b0bd660bbf05debe472e/pandas/tests/extension/base/methods.py) !

 - There are no PEP8 issues in the file [`pandas/tests/extension/base/ops.py`](https://github.com/mwoss/pandas/blob/f1f1fb76b903ad893065b0bd660bbf05debe472e/pandas/tests/extension/base/ops.py) !

 - There are no PEP8 issues in the file [`pandas/tests/extension/base/reduce.py`](https://github.com/mwoss/pandas/blob/f1f1fb76b903ad893065b0bd660bbf05debe472e/pandas/tests/extension/base/reduce.py) !

 - There are no PEP8 issues in the file [`pandas/tests/extension/base/reshaping.py`](https://github.com/mwoss/pandas/blob/f1f1fb76b903ad893065b0bd660bbf05debe472e/pandas/tests/extension/base/reshaping.py) !

 - There are no PEP8 issues in the file [`pandas/tests/extension/base/setitem.py`](https://github.com/mwoss/pandas/blob/f1f1fb76b903ad893065b0bd660bbf05debe472e/pandas/tests/extension/base/setitem.py) !

 - There are no PEP8 issues in the file [`pandas/tests/extension/decimal/array.py`](https://github.com/mwoss/pandas/blob/f1f1fb76b903ad893065b0bd660bbf05debe472e/pandas/tests/extension/decimal/array.py) !

 - There are no PEP8 issues in the file [`pandas/tests/extension/decimal/test_decimal.py`](https://github.com/mwoss/pandas/blob/f1f1fb76b903ad893065b0bd660bbf05debe472e/pandas/tests/extension/decimal/test_decimal.py) !

 - There are no PEP8 issues in the file [`pandas/tests/extension/json/array.py`](https://github.com/mwoss/pandas/blob/f1f1fb76b903ad893065b0bd660bbf05debe472e/pandas/tests/extension/json/array.py) !

 - There are no PEP8 issues in the file [`pandas/tests/extension/json/test_json.py`](https://github.com/mwoss/pandas/blob/f1f1fb76b903ad893065b0bd660bbf05debe472e/pandas/tests/extension/json/test_json.py) !

 - There are no PEP8 issues in the file [`pandas/tests/extension/test_categorical.py`](https://github.com/mwoss/pandas/blob/f1f1fb76b903ad893065b0bd660bbf05debe472e/pandas/tests/extension/test_categorical.py) !

 - There are no PEP8 issues in the file [`pandas/tests/extension/test_common.py`](https://github.com/mwoss/pandas/blob/f1f1fb76b903ad893065b0bd660bbf05debe472e/pandas/tests/extension/test_common.py) !

 - There are no PEP8 issues in the file [`pandas/tests/extension/test_external_block.py`](https://github.com/mwoss/pandas/blob/f1f1fb76b903ad893065b0bd660bbf05debe472e/pandas/tests/extension/test_external_block.py) !

 - There are no PEP8 issues in the file [`pandas/tests/extension/test_integer.py`](https://github.com/mwoss/pandas/blob/f1f1fb76b903ad893065b0bd660bbf05debe472e/pandas/tests/extension/test_integer.py) !

 - There are no PEP8 issues in the file [`pandas/tests/extension/test_interval.py`](https://github.com/mwoss/pandas/blob/f1f1fb76b903ad893065b0bd660bbf05debe472e/pandas/tests/extension/test_interval.py) !

 - There are no PEP8 issues in the file [`pandas/tests/extension/test_sparse.py`](https://github.com/mwoss/pandas/blob/f1f1fb76b903ad893065b0bd660bbf05debe472e/pandas/tests/extension/test_sparse.py) !

 Some tests not passing due to lack of pyarrow library in project dependencies, What should I do? After adding pytest.importorskip again, build is still failing. (probably PEP8 code style issue) `isort` thought these two are not properly sorted. 
```
ERROR: /home/travis/build/pandas-dev/pandas/pandas/tests/extension/arrow/bool.py Imports are incorrectly sorted.
ERROR: /home/travis/build/pandas-dev/pandas/pandas/tests/extension/arrow/test_bool.py Imports are incorrectly sorted.
```
In my case, it was sorted but `code_checks.sh` wasn't working properly. So you could exclude and add them back to setup.cfg and let Jeff knows. That was what I did.   thanks @mwoss  @mwoss 
👏 