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

[Craig Hesling via llvm-commits]

linux4life798 wrote:

Looks like we may have hit the first python version issue:

>From the [Window x64 buildkit job](https://buildkite.com/llvm-project/github-pull-requests/builds/33515#018d492c-fee4-4595-a747-7655e3a85b41):

```
-- CTTestTidyModule ignored -- Loadable modules not supported on this platform.
-- LLD version: 19.0.0
-- Performing Test C_SUPPORTS_WERROR_IMPLICIT_FUNCTION_DECLARATION
-- Performing Test C_SUPPORTS_WERROR_IMPLICIT_FUNCTION_DECLARATION - Failed
-- Performing Test C_SUPPORTS_WERROR_MISMATCHED_TAGS
-- Performing Test C_SUPPORTS_WERROR_MISMATCHED_TAGS - Failed
-- Found Python3: C:/Python39/python.exe (found suitable version "3.9.7", minimum required is "3.8") found components: Interpreter Development Development.Module Development.Embed
CMake Error at C:/BuildTools/Common7/IDE/CommonExtensions/Microsoft/CMake/CMake/share/cmake-3.20/Modules/FindPackageHandleStandardArgs.cmake:230 (message):
  Could NOT find Python3 (missing: Python3_NumPy_INCLUDE_DIRS NumPy) (found
  suitable version "3.9.7", minimum required is "3.8")
Call Stack (most recent call first):
  C:/BuildTools/Common7/IDE/CommonExtensions/Microsoft/CMake/CMake/share/cmake-3.20/Modules/FindPackageHandleStandardArgs.cmake:594 (_FPHSA_FAILURE_MESSAGE)
  C:/BuildTools/Common7/IDE/CommonExtensions/Microsoft/CMake/CMake/share/cmake-3.20/Modules/FindPython/Support.cmake:3165 (find_package_handle_standard_args)
  C:/BuildTools/Common7/IDE/CommonExtensions/Microsoft/CMake/CMake/share/cmake-3.20/Modules/FindPython3.cmake:485 (include)
  C:/ws/src/mlir/cmake/modules/MLIRDetectPythonEnv.cmake:21 (find_package)
  C:/ws/src/mlir/CMakeLists.txt:172 (mlir_configure_python_dev_packages)
-- Configuring incomplete, errors occurred!
See also "C:/ws/src/build/CMakeFiles/CMakeOutput.log".
See also "C:/ws/src/build/CMakeFiles/CMakeError.log".
```

It appears to be indicating that the version of Python is fine (3.9.7), but it couldn't find a suitable version of **NumPy**.

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