CLN: Old string formatting: .format() -> f
- [ ] ref #29547
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Hello @DorAmram! 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 2019-12-22 18:03:07 UTC Hmm I think this file is autogenerated as per @jbrockmendel comments here https://github.com/pandas-dev/pandas/pull/29518#discussion_r344826266 yeah we can leave this one alone, do we have a check for .format usage in checks? (prob not yet) > yeah we can leave this one alone, do we have a check for .format usage in checks? (prob not yet)

Not yet there are still 200+ files to tackle. 
`grep -l -R -e '%s' -e '%d' -e '\.format(' --include=*.{py,pyx} pandas/`

If we want to add a check now we can do - and users remove the file from exclude list once done?
(Would help confirm all instances of .format have been removed in the PR for a file)
 grepping for `.format(` will be complicated by the fact that we have 6 places where we `def format\(`.  But we can worry about that later.  This PR is non-actionable, closing.