The ornamental design for a seat, as shown and described.
FIG. 1 shows a perspective view of the seat in accordance with the new design.
FIG. 2 shows a front elevation view of the seat.
FIG. 3 shows a rear view of the seat.
FIG. 4 shows a top view of the seat; and,
FIG. 5 shows a side view of the seat.
The broken line showing of stitching on the cushions, arms, and back of the seat illustrates stitching that forms a part of the claimed design. All other broken lines illustrate portions of the seat that form no part of the claimed design.