Stop using flynt

[eerovaher]

Description

flynt is a tool that can replace %-formatted strings and str.format() calls with f-strings, but in practice it doesn't seem to be doing anything ruff couldn't do.

Closes #14934

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[pllim]

@WilliamJamieson added this in #14278 so I want to see if he has objection. But if you do not hear back in a week or two, feel free to merge. Thanks!

[neutrinoceros]

We're a week or two later 👀