fix flake8 file syntax, because 5->6
fix flake8 file syntax, because 5->6

https://stackoverflow.com/questions/74558565/flake8-error-code-supplied-to-ignore-option-does-not-match-a-z1-30
https://flake8.pycqa.org/en/latest/release-notes/6.0.0.html#id1

hopefully it works
open2c/cooltools