TST: Filter false positive matplotlib warning
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).

xref https://github.com/matplotlib/matplotlib/issues/23614.

This false positive warning showed up in our test suite and can be filtered out.

thx @mroeschke 