libvirt-python: 3.6.0 -> 3.8.0

This fixes an eval error introduced by
17a06927

.

The reason the evaluation fails is because there's an assertion on the
version of the Python library in order to be in par with the top-level
libvirt package.

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @fpletz