CLN: clean up new detected trailing whitespace
- [N/A] closes #xxxx (xref [#36386 (comment)](https://github.com/pandas-dev/pandas/pull/36386#discussion_r493865987))
- [N/A] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [N/A] whatsnew entry

Fixed the remaining trailing whitespace problems that weren't being found by `ci/code_checks.sh` because of the file type filters.
