The ornamental design for a paintbrush, as shown and described.
FIG. 1 is a perspective view of a paintbrush showing my new design;
FIG. 2 is a front elevational view of the paintbrush shown in FIG. 1, the
rear elevational view and the top and bottom plan views being the same as
the front elevational view;
FIG. 3 is a right side elevational view of the paintbrush shown in FIG. 1;
and,
FIG. 4 is a left side elevational view of the paintbrush shown in FIG. 1.