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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> I think #208773 will introduce another Python use -- no idea if that's a good idea or not.

That could be rewritten in cmake, but it's a question of whether anyone would rather maintain cmake or python. Writing a unit test for cmake seems like a pain too 

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