[llvm] [2/4]: Bump minimum required sphinx Python to 3.8 (PR #203963)

[James Henderson via llvm-commits]

jh7370 wrote:

LLVM's [documented policy](https://llvm.org/docs/GitHub.html#updating-pull-requests) is that force pushes should be avoided. I've yet to come across where a force push is actually required, because you can always merge from main/your base branch into the branch for the PR, which has the same impact in terms of diff in GitHub, without trashing the review history. As we do Squash & Merge at the end, the merge commit disappears.

https://github.com/llvm/llvm-project/pull/203963