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

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 commented:

There are a couple other files that should probably be updated:
* `llvm/docs/GettingStartedVS.rst`
* `llvm/docs/TestingGuide.rst`

That looks like it should be it when I searched for `3.6` within the repository.

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