Revert "CMake: Replace deprecated module FindPythonInterp"

Revert "CMake: Replace deprecated module FindPythonInterp"

This reverts commit d6380e7ae40ec53cbd9119e9527546656f82d660.

Turns out we were unwittingly still using FindPythonInterp
instead of FindPython3.cmake, via LocatePythonModule.cmake,
nd this commit actually enabled FindPython3.cmake. Also turns
out FindPython3.cmake is far too clever and very buggy with MSYS2.
It will usually not find the correct python binary and fail in many
suprising ways, depending on which combination of Python Windows
installations is present.