Slicing columns with mixed types <str>,<int> fails with ValueError #20975
- [ ] closes #20975 
- [x] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Hello @hungyiwu! 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 2020-12-27 13:43:36 UTC I saw only the travis-ci check failed but when going through its log I couldn't identify the cause. Any suggestions will be appreciated! thanks @hungyiwu 