CLN: changed .format to fstring in pandas/io/formats/csvs.py

[sardonick]

• closes #xxxx
• tests added / passed
• passes black pandas
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

#29547

My first contribution.

[alimcmaster1]

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 

[alimcmaster1]

Issue to ref here is #29547

[pep8speaks]

Hello @sardonick! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

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

Comment last updated at 2019-12-24 01:38:19 UTC

[jreback]

thanks @sardonick