[llvm] Revert "[CMake] Use minimum python version of 3.8 everywhere" (PR #209233)

[Nikita Popov via llvm-commits]

nikic wrote:

> I'm wondering if we should just drop the python requirement entirely. The only function inside of CMake that seems to use it is `add_llvm_symbol_exports` that could probably be rewritten to work without it.

I think https://github.com/llvm/llvm-project/pull/208773 will introduce another Python use -- no idea if that's a good idea or not.

https://github.com/llvm/llvm-project/pull/209233