[llvm] [python] Bump Python minimum version to 3.8 (PR #78828)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

Getting buildbots updated and relanding the change and potentially repeating that process is the next step. I've opened https://github.com/llvm/llvm-project/issues/83962 and emailed buildbot maintainers for bots that failed after landing this change due to old Python versions.

https://github.com/llvm/llvm-project/pull/78828