The ornamental design for a costume, as shown and described.
FIG. 1 is a front elevation view of a costume showing the new design;
FIG. 2 is a rear elevation view thereof;
FIG. 3 is a left side elevation view thereof;
FIG. 4 is a right side elevation view thereof;
FIG. 5 is a front perspective view thereof; and,
FIG. 6 is a front bottom perspective view thereof.
The broken lines in the drawings form a boundary of the claim, indicating that the design does not include the area of the head opening or the letters “T” and “M” on the front left of the torso, and form no part of the claimed design.