CMake: make sure that we can find Python 3.8.x.
CMake: make sure that we can find Python 3.8.x.

cellml/libcellml,cellml/libcellml,hsorby/libcellml,hsorby/libcellml,cellml/libcellml,hsorby/libcellml,nickerso/libcellml,hsorby/libcellml,cellml/libcellml,nickerso/libcellml,nickerso/libcellml,nickerso/libcellml