Fix PR08, RT02, RT03, and SA01 on pandas.Index.fillna
- [x] closes #https://github.com/pandanistas/pandanistas_sprint_ui2020/issues/5
- [ ] tests added / passed
- [x] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
