The ornamental design for a bathtub, as shown and described.
FIG. 1 is a perspective view of a bathtub showing our 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 are directed to environment and are for illustrative purposes only; the broken lines form no part of the claimed design.