CLN: changed .format to fstring in pandas/io/formats/csvs.py
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

https://github.com/pandas-dev/pandas/issues/29547

My first contribution.