The ornamental design for a paintbrush handle, as shown and described.
FIG. 1 is an upper right rear perspective view of my paint brush handle design, the bristles and bristle holder are shown in phantom lines and are not claimed as part of my design;
FIG. 2 is a left side elevational view of my design, the bristles and bristle holder are shown in phantom lines and are not claimed as part of my design, the right side is a mirror image thereof;
FIG. 3 is a front elevational view of my design, the area denoted by dashed lines is not a part of my design;
FIG. 4 is a rear elevational view of my design;
FIG. 5 is a top plan view of my design, the bristles and bristle holder are shown in phantom lines and are not claimed as part of my design; and,
FIG. 6 is a bottom plan view of my design, the bristles and bristle holder are shown in phantom lines and are not claimed as part of my design.