Set legacy python parameters for VTK's FindPythonLibs.
Currently, VTK's FindPythonLibs.cmake sets the above variables, but CMake's FindPythonLibs.cmake does not. An issue has been made for VTK to update its FindPythonLibs to reflect the parameters of the newer CMake version (see vtk/vtk#16848 (closed)). Until this is resolved, we manually set these legacy parameters.