The ornamental design for sharpening tool, as shown and described.
FIG. 1 is a perspective view of the sharpening tool embodying the new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
FIGS. 1-7 show features of a sharpening tool in broken lines. These features are shown for environmental purposes and do not form any part of the claimed design. The broken lines immediately adjacent the solid line portions of the design form the boundary of the design, with the broken lines forming no part of the claimed design.