Update builddep command in BUILD.txt
    
    It's no longer necessary to specify "with_python3" to get Python 3
    dependencies.
    
    python3-tox pulls in Python 2.6, 3.3, 3.4, 3.5, and pypy as weak
    dependency. Use --setopt=install_weak_deps=False to make a build
    environment leaner.
    
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>