vm/syslinux: Drop the Python 2 dependency

Part of #286 (closed)

This has got to be the easiest port "away from Python 2" I ever did. 🤣

To make sure I didn't break anything, I did the following:

$ bst build all.bst
$ bst build vm/minimal-systemd-vm-image-x86_64.bst
$ bst checkout vm/minimal-systemd-vm-image-x86_64.bst fdo-image-x86_64
$ cd fdo-image-x86_64
$ ./fdo-image-x86_64/run-in-qemu.sh

That booted the VM to a login shell.

I tried importing the disk image in GNOME Boxes, which worked just as well.