CLN: Add typing for dtype argument in io directory (GH38808)
incremental PR for issue #38808
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

thanks @avinashpancham 