The ornamental design for a seat, as shown and described.
1.1is a top, front and side perspective view of a seat;
1.2is a top, rear and opposite side view thereof, with an additional front view of multiple seats stacked one on top of another. The disclosure of multiple seats in FIG. 1.2is to show the seat of the invention in a stackable configuration. The additional seats shown in the figure are not part of the claimed design.