The ornamental design for a toothbrush, as shown and described.
FIG. 1 is a front perspective view of a toothbrush according to the new design;
FIG. 2 is a rear perspective view thereof;
FIG. 3 is a front elevation view thereof;
FIG. 4 is a rear elevation view thereof;
FIG. 5 is a left side elevation view thereof;
FIG. 6 is a right side elevation view thereof;
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan view thereof.
The broken lines illustrating portions of the oral care implement form no part of the claimed design. The dot-dash broken lines define the bounds of the claimed design and form no part thereof.