The ornamental design for a toothbrush, as shown and described.
FIG. 1 is a perspective view of a toothbrush showing my new design;
FIG. 2 is an enlarged top view thereof;
FIG. 3 is an enlarged bottom view thereof;
FIG. 4 is an enlarged right-side view thereof, the left-side view being a mirror-image;
FIG. 5 is an enlarged front view thereof;
FIG. 6 is an enlarged rear view thereof; and,
FIG. 7 is an enlarged, partial, front view thereof.
The portions of the toothbrush shown in broken lines form no part of the claimed design. The broken lines that are immediately adjacent the surface shading represent the bounds of the claim and form no part thereof.