The ornamental design for a toothbrush, as shown and described.
FIG. 1 is a front left perspective view of a toothbrush according to the new design;
FIG. 2 is a front right perspective view thereof;
FIG. 3 is a rear right perspective view thereof;
FIG. 4 is a rear left perspective view thereof;
FIG. 5 is a front elevation view thereof;
FIG. 6 is a rear elevation view thereof;
FIG. 7 is a left side elevation view thereof;
FIG. 8 is a right side elevation view thereof;
FIG. 9 is an enlarged top plan view thereof; and,
FIG. 10 is an enlarged 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.