Revert "python: use vendored tomli"

Now that Ubuntu 20.04 is not included anymore, there is no need to ship
it as part of QEMU; Ubuntu 22.04 includes it and Leap users anyway
need to install all the required dependencies from PyPI.

This mostly reverts commit ec77ee7634de123b7c899739711000fd21dab68b,
with just some changes to the wording.

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