The ornamental design for a toothbrush, as shown and described.
FIG. 1 is a top-front perspective view of a toothbrush showing my new design;
FIG. 2 is a bottom-rear perspective view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a right elevational view thereof;
FIG. 5 is a rear elevational view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof;
FIG. 8 is a cross sectional right view taken from FIG. 4 thereof; and,
FIG. 9 is an exploded front elevational view thereof.
The broken line showing the brush head and the suction cup in FIGS. 1, 2, 3, 4, 5, 6, 7, 8, and 9 are included for the purpose of illustrating environmental structure and forms no part of the claimed design.