The ornamental design for a bathtub, as shown and described.
FIG. 1 is a front perspective view of a bathtub showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left-side elevational view thereof;
FIG. 5 is a right-side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines represent environmental structure, surface contour, and/or, if immediately adjacent the shaded areas, the bounds of the claimed design. The broken lines themselves form no part of the claimed design.