The ornamental design for a paintbrush, as shown and described.
FIG. 1 is a perspective view of the paint brush.
FIG. 2 is a front view thereof.
FIG. 3 is a back view thereof.
FIG. 4 is a left side view thereof.
FIG. 5 is a right side view thereof.
FIG. 6 is a bottom view thereof.
FIG. 7 is a top view thereof; and,
FIG. 8 is a partial sectional view of the paint brush illustrating a connecting portion of the paint brush. All other views of the connecting portion are identical to the view showed in FIG. 8.
The broken lines present in the drawings represent the environment and form no part of the claimed design.