py3 spec: remove python2 dependencies from server-trust-ad
    
    Use only python3 dependencies when building server-trust-ad for python3.
    
    Fixes: https://pagure.io/freeipa/issue/7208
    Signed-off-by: Tomas Krizek <tkrizek@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>