Switch Fedora 29 and rawhide builds to python3/gtk3

Switch to python3 and gtk3. Also enable scripting.

I've built this for f28, f29, and rawhide to test the "ifdefs". For f28 it should be unchanged - still uses python2.

However, I'm not sure we want to merge this yet. I see some strange line widths at some zoom levels.

I'd appreciate feedback as to whether anyone else is seeing that.