The ornamental design for an toothbrush, as shown and described.
FIG. 1 is a front left perspective view of an embodiment of a toothbrush showing 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 are provided for environment only and form no part of the claimed design. The broken lines formed by a dash-dot-dash sequence represent an unclaimed boundary.