The ornamental design for a seat, as shown and described.
FIG. 1 is a perspective view of a seat showing my new design;
FIG. 2 is a first side elevational view thereof;
FIG. 3 is a second side elevational view thereof;
FIG. 4 is a third side elevational view thereof;
FIG. 5 is a fourth side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines shown in the drawings, including those representing stitching are included for the purpose of illustrating elements of the seat that form no part of the claimed design.