BUG: Fix to_excel writers handling of cols
- [x] closes #31677
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry - pending https://github.com/pandas-dev/pandas/pull/31723

Ref: https://github.com/pandas-dev/pandas/pull/31723

cc. @WillAyd @jbrockmendel 