The ornamental design for a seat, as shown and described.
FIG. 1 is an isometric view of the seat of my new design having two seating sections;
FIG. 2 is a left side elevational view thereof, the right side elevational view being a mirror image thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a back elevational view thereof; and,
FIG. 5 is an isometric view of a second embodiment of the seat of my new design which differs from the first embodiment only by the addition of another seating section.