I claim the ornamental design for a bathtub, as shown and described.
FIG. 1 is a downward perspective view of a bathtub.
FIG. 2 is a forward perspective view of a bathtub.
FIG. 3 is a top plan view of a bathtub.
FIG. 4 is a bottom plan view of a bathtub.
FIG. 5 is a front elevation and a rear elevation view of a bathtub; and,
FIG. 6 is a right side and a left side elevation view of a bathtub.
In the drawings, the broken lines depict portions of the bathtub that form no part of the claimed design.
The boundry lines, comprising dash/dot lines in FIGS. 1, 2 and 4-6, indicate the area between claimed and unclaimed portions of the bathtub and form no part of the claimed design.