fix crash on macOS when opening OpenSCAD Workbench
fix crash on macOS when opening OpenSCAD Workbench

The OpenSCAD Workbench crashes on open on macOS with error
"a bytes-like object is required, not 'str'", due to python 2 to
python 3 incompatibility. This is me implementing the fix described by
oapa at
https://forum.freecadweb.org/viewtopic.php?t=35384#p311908 (with an
untested attempt to make it also still run with python 2).

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