The ornamental design for a litterbox, as shown and described.
FIG. 1 is a perspective, front and side view of the litter box;
FIG. 2 is a front view of the litter box;
FIG. 3 is a rear view of the litter box;
FIG. 4 is a left side view of the litter box;
FIG. 5 is a right side view of the litter box;
FIG. 6 is a top plan view of the litter box; and,
FIG. 7 is a bottom view of the litter box.
The broken lines in the drawings illustrate portions of the litter box that form no part of the claimed design. The dot-dash lines in the drawings illustrate contours of the litter box that form no part of the claimed design.