The ornamental design for a toothbrush, as shown and described.
1. Toothbrush
1.1 is a top perspective view;
1.2 is a bottom perspective view;
1.3 is a front view, the back view being a mirror image;
1.4 is a left view;
1.5 is a right view;
1.6 is a top view; and
1.7 is a bottom view.
In the reproductions, the broken lines are for the purpose of illustrating portions of the toothbrush which form no part of the claimed design. The thin lines in the views are for clarifying the shape of the three-dimensional surface.