Declare Python 3 support and prefer it in packaging
    
    Now that Pagure supports Python 3 and it appears to work across the
    board, we should prefer it by default and package for Python 3 whenever
    it is reasonably possible.
    
    Also, since we dropped EL6 support, Python 2.6 support is also dropped.
    
    Signed-off-by: Neal Gompa <ngompa13@gmail.com>