gh-129370: Update libregrtest to use f-strings

[lczyk]

Found and fixed all instances with ruff (And did read through them as opposed to just submitting a PR with a tool result! 😉):

ruff check --select UP031,UP032 --fix --unsafe-fixes ./Lib/test/libregrtest/

I've omitted the changes to Regtest::run_tests_sequentially as to not conflict with #129363.

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

• Issue: libregrtest f-string update #129370

[bedevere-app]

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[ZeroIntensity]

Per @encukou closing the issue, we don't like to accept cosmetic changes like this. Sorry :(

[lczyk]

@ZeroIntensity ok 👍 good to know. sorry for the noise

[ZeroIntensity]

It does hurt on our end to close PRs that had effort go into them, so hopefully this isn't discouraging! If you're interested in contributing, you might want to take a look at the devguide, which will help guide you through making PRs that get accepted.