The ornamental design for a toothbrush, as shown and described.
FIG. 1 is a front view of a toothbrush showing our new design;
FIG. 2 is a right side view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a bottom view thereof;
FIG. 6 is a front perspective view thereof; and,
FIG. 7 is a rear perspective view thereof.
The broken line showing of the top portion of the handle and the toothbrush head is for illustrative purposes only and forms no part of the claimed design.