TST: Series.update with categorical
- [ ] closes #25744, closes #27338
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Hello @ftrihardjo! 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 2021-01-03 02:14:22 UTC Hi @ftrihardjo - could you please put a meaningful title and link to the issue you're closing in the pull request body? see here: https://github.com/pandas-dev/pandas/pull/38873/checks?check_run_id=1634842619

make sure you have `precommit` installed it will fix anything when you commit thanks @ftrihardjo 