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

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> 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

Agreed. I think we probably just reland this. The getting started page already implies we unconditionally require python 3.8.

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