The ornamental design for a bathtub, as shown and described.
FIG. 1 is a perspective view of a bathtub showing my new design;
FIG. 2 is a front elevational view of the bathtub shown in FIG. 1;
FIG. 3 is a rear elevational view of the bathtub shown in FIG. 1;
FIG. 4 is a left-side elevational view of the bathtub shown in FIG. 1;
FIG. 5 is a right-side elevational view of the bathtub shown in FIG. 1;
FIG. 6 is a top plan view of the bathtub shown in FIG. 1; and,
FIG. 7 is a bottom plan view of the bathtub shown in FIG. 1.
The broken lines immediately adjacent the shaded areas represent the bounds of the claimed design, while all other broken lines depict portions of the article that form no part of the claimed design.