[Libguestfs] [PATCH nbdkit] python: Drop support for Python 2.

This patch proposes to drop support for Python 2 in nbdkit. Rather than abruptly drop it everywhere, my proposal is that we point people to nbdkit 1.14 (the current stable version) if they want to continue with Python 2 plugins, while gently reminding them of the upcoming Python 2.7 end of life announcement. Libnbd never supported Python 2. Libguestfs in theory supports Python 2 but I dropped the bindings from Fedora back in March so it's basically never tested now. Libvirt also started to prefer Python 3 over Python 2 back in March. Rich.