TST: Add tests for numeric_only default behaviour (GH41351)
- [x] closes #41351
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them

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



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2021-06-10 20:50:13 UTC Thanks @michaelneururer for the PR. The added test is failing ci. @simonjayhawkins Oh, I thought that the issue was resolved on the master branch of pandas. I didn't realise that you run tests with various different numpy versions. The failure I reported in #41351 depends on the numpy version used (it works from numpy version 1.20.2). 

I am not sure I have the time to fix the issue if an older version of numpy is used. Could you advise on what I should do now? This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this. Thanks for the PR, but as you discovered, this PR would need a fix that depends on the numpy version in addition to tests. If you're still interested in investigating a fix in this case, let us know and we can reopen this PR, but closing for now.