The ornamental design for a costume, as shown and described.
FIG. 1 is a front elevational view of my new design;
FIG. 2 is a side elevational view thereof, with the opposite side being a mirror image of the same;
FIG. 3 is a rear elevational view thereof; and,
FIG. 4 is a top plan view thereof.
The broken lines form no part of the claimed design. In particular, the broken lines that show the wrinkles or creases formed in the fabric of the costume form no part of the claimed design.