FEM: nonlinear material fix for Py2 and reactivate unit test
FEM: nonlinear material fix for Py2 and reactivate unit test

Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master