The ornamental design for a toothbrush, as shown and described.
FIG. 1 is a front view of the article in a stored configuration.
FIG. 2 is a right side view of the article in a stored configuration.
FIG. 3 is a bottom view of the article in a stored configuration.
FIG. 4 is a rear view of the article in a stored configuration.
FIG. 5 is a top view of the article in a stored configuration.
FIG. 6 is a left side view of the article in a stored configuration.
FIG. 7 is a perspective view of the article in a stored configuration; and,
FIG. 8 is a perspective view of the article in an in use configuration.
If portions of the structures shown in the drawings include broken lines, such portions represent environment(s) in which the claimed design may be used.