CLN: String formatting % -> f-strings
- Follow up from https://github.com/pandas-dev/pandas/pull/29212
- Towards https://github.com/pandas-dev/pandas/issues/29547

- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Old style % formatting to f-strings
looks like black complaints @jbrockmendel - ready to re-review whenever you have a moment  A handful of comments, general rule about minimizing the code that is executed inside f-space.  If its viable to have the black-version-related stuff not in this diff that'll make it a little easier.  cc @gfyoung for another look Looks pretty good!  I agree with the comments here. Updated as per comments and green cc. @jbrockmendel and @gfyoung  thanks @alimcmaster1 