[llvm] [utils] update type hint for python 3.8-3.9 compatibility (PR #183427)

[Henrik G. Olsson via llvm-commits]

hnrklssn wrote:

> It might be better to just start using the modern `list[foo]` that doesn't need typing imports and use `from __future__ import annotations` which I believe has worked since 3.7

good suggestion! addressed in https://github.com/llvm/llvm-project/pull/184225

https://github.com/llvm/llvm-project/pull/183427