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.
Test failures related to @MomIsBestFriend comments above:

```
 with pytest.raises(ValueError, match=msg):
>               float_frame.to_csv(path, header=["AA", "X"])
E               AssertionError: Pattern 'Writing 4 cols but got 2 aliases' not found in 'Writing 4 cols but got 
``` Issue to ref here is https://github.com/pandas-dev/pandas/issues/29547 Hello @sardonick! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2019-12-24 01:38:19 UTC thanks @sardonick 