Debian only introduced tomli in the bookworm release.  Use a
vendored wheel to avoid requiring a package that is only in
bullseye-backports and is also absent in Ubuntu 20.04.

While at it, fix an issue in the vendor.py scripts which does
not add a newline after each package and hash.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>