TST: Add test to Series.str.strip()
- [x] closes #44867
- [x] tests added
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [ ] whatsnew entry

Hello @minnaau! Thanks for opening this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:

* In the file [`pandas/tests/strings/test_split_partition.py`](https://github.com/pandas-dev/pandas/blob/101de9f476fbe7743736d43558cf52d33c2180c2/pandas/tests/strings/test_split_partition.py):

> [Line 111:1](https://github.com/pandas-dev/pandas/blob/101de9f476fbe7743736d43558cf52d33c2180c2/pandas/tests/strings/test_split_partition.py#L111): [E302](https://duckduckgo.com/?q=pep8%20E302) expected 2 blank lines, found 1
> [Line 126:1](https://github.com/pandas-dev/pandas/blob/101de9f476fbe7743736d43558cf52d33c2180c2/pandas/tests/strings/test_split_partition.py#L126): [W293](https://duckduckgo.com/?q=pep8%20W293) blank line contains whitespace
> [Line 131:1](https://github.com/pandas-dev/pandas/blob/101de9f476fbe7743736d43558cf52d33c2180c2/pandas/tests/strings/test_split_partition.py#L131): [W293](https://duckduckgo.com/?q=pep8%20W293) blank line contains whitespace
> [Line 136:1](https://github.com/pandas-dev/pandas/blob/101de9f476fbe7743736d43558cf52d33c2180c2/pandas/tests/strings/test_split_partition.py#L136): [W293](https://duckduckgo.com/?q=pep8%20W293) blank line contains whitespace
> [Line 141:1](https://github.com/pandas-dev/pandas/blob/101de9f476fbe7743736d43558cf52d33c2180c2/pandas/tests/strings/test_split_partition.py#L141): [W293](https://duckduckgo.com/?q=pep8%20W293) blank line contains whitespace
> [Line 146:1](https://github.com/pandas-dev/pandas/blob/101de9f476fbe7743736d43558cf52d33c2180c2/pandas/tests/strings/test_split_partition.py#L146): [W293](https://duckduckgo.com/?q=pep8%20W293) blank line contains whitespace

 Thanks @minnaau for the PR.

IIRC we have plenty of tests for the string methods on mixed object Series.

In the issue there is a suggestion that the docs could be updated to clarify the behavior.

So closing this PR as there is another PR open #44885 that is updating the docs and IMO we don't really need any additional tests.