[clang] [clang][Python] Use fstrings instead of string concatenations (PR #173861)

[Vlad Serebrennikov via cfe-commits]

https://github.com/Endilll requested changes to this pull request.

I'm not sure this is an improvement overall, because format of strings is obscured by the code inside curly braces. If we are to make changes here, I'd rather reach for `string.format()`

https://github.com/llvm/llvm-project/pull/173861