You can make your very own custom viewport. It may have black bars, and it may or may not keep it's aspect ratio, depending on how you program it. A custom viewport would look something like this:
public class Tutorial extends Viewport. You would need to override
update(width, height), but that's it.
Another attempt would be to extend the generic
ScalingViewport, and supplying another Scaling which is not already supplied. Suppose that you set it to
Scaling.none. That would look something like this:
If you want some reference, you can find the builtin viewport classes right over here.