BUG: repair 'style' kwd handling in DataFrame.plot (#21003)
- [x] closes #21003
- [x] tests added / passed
- [x] passes `black pandas`
    => `command not found`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
    => `command not found`
- [x] whatsnew entry
