The ornamental design for a swimmingcostume, as shown and described.
FIG. 1 is a front elevational view of the swimming costume showing my new design;
FIG. 2 is a rear elevational view thereof;
FIG. 3 is a left side elevational view thereof;
FIG. 4 is a right side elevational view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a bottom plan view thereof;
FIG. 7 is a perspective view thereof shown in a position of use; and,
FIG. 8 is another perspective view thereof shown in a position of use.
The broken lines shown in the drawings illustrate stitching, portions of the swimming costume, and environment that form no part of the claimed design.