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

[Craig Hesling via llvm-commits]

linux4life798 wrote:

> This need a release note as well!

I see. So, probabloy adding a note to the [Other Changes section of LLVM](https://llvm.org/docs/ReleaseNotes.html#other-changes) and to the [Python Binding section of clang](https://llvm.org/docs/ReleaseNotes.html#changes-to-the-python-bindings)?

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