The ornamental design for a costume, as shown and described.
FIG. 1 is a perspective view of a costume showing my new design;
FIG. 2 is a front view of the costume shown in FIG. 1;
FIG. 3 is a left side view of the costume shown in FIG. 1; and,
FIG. 4 is a right side view of the costume shown in FIG. 1.
The broken lines, which represent the environmental structure of a jumpsuit and connectors, form no part of the claimed design. All other surface(s) and portion(s) of the costume not shown in the figures or otherwise described in the specification form no part of the claimed design.