CLN: Linting tailing whitespaces, and fixing remaining cases
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

@jreback I guess this makes sense. Not sure if there is any reason to exclude html or csv files, let me know if that's the case.