The ornamental design for a toothbrush, as shown and described.
FIG. 1 is a front right perspective view of an embodiment of a toothbrush showing the new design;
FIG. 2 is a rear left perspective view thereof;
FIG. 3 is a front elevation view thereof;
FIG. 4 is a rear elevation view thereof;
FIG. 5 is a right side elevation view thereof;
FIG. 6 is a left side elevation view thereof;
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan view thereof.
The broken lines formed by linear segments of equal length illustrate portions of the toothbrush that form no part of the claimed design. The broken lines formed by a dash-dot-dash sequence represent an unclaimed boundary.