The ornamental design for a settopbox, as shown and described.
FIG. 1 is a front perspective view of a set top box showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken lines immediately adjacent to and surrounding the shaded area represent the bounds of the claimed design and form no part thereof. The broken lines depicting the buttons, outlet openings, and remainder of the set top box form no part of the claimed design.