CLN: Orderdict -> defaultdict(list)
- [x] ref #30469
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

> lgtm, thanks for cleaning this @MomIsBestFriend

Thanks for the detailed explanation, and the patience ;) thanks @MomIsBestFriend 