spec: Fix kpatch-dnf dependencies
    
    Unlike kpatch and kpatch-dnf, python3 is not part of the BaseOS compose.
    Hence, kpatch-dnf shouldn't depend on python3. Akin to other BaseOS dnf
    plugins, depending on python3-dnf is enough and have a dependency on
    system-python to have python3 ABI. (And kpatch-dnf is not shipped on
    RHEL version having system-python with python2 ABI, so we don't have to
    worry about that case)
    
    Resolves: rhbz#1912457
    Signed-off-by: Julien Thierry <jthierry@redhat.com>