CLN: GH29547 format with f-strings
xref #29547 
replace .format() for f-strings in the following:

- pandas/tests/series/indexing/test_numeric.py

Hello @DanBasson! 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 2020-06-21 17:03:13 UTC hi @jreback, 

do you understand the error ?
any help will be appriciated.
CI is successful.