The ornamental design for a swimmingcostume, as shown and described.
1.1: Front outside;
1.2: Back outside;
1.3: Front inside; and
1.4: Back inside.
The designated dashed - - dashed lines illustrated in FIGS. 1.1, 1.2, and 1.4 indicate stitching on the bathing suit. The staggered dashed dashed lines surrounding the zipper in FIG. 1.2 indicates a stitching pattern on the bathing suit. Those solid lines shown on the inside of the bathing suit in FIGS. 1.3 and 1.4 not illustrated on the outside of the bathing suit in FIG. 1.1 or 1.2 are features that cannot be seen from outside of the bathing suit. Those remaining portions of the Figures shown in broken lines are for illustration only and form no part of the claimed design.