The ornamental design for a stepstool, as shown and described.
FIG. 1 is a front, right, and top perspective view of a step stool, showing my design.
FIG. 2 is a rear, right, and bottom perspective view thereof.
FIG. 3 is a front elevation view thereof.
FIG. 4 is a rear elevation view thereof.
FIG. 5 is a left side elevation view thereof.
FIG. 6 is a right side elevation view thereof.
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan view thereof.
The broken lines shown in the drawings are included for the purpose of illustrating portions of the step stool that form no part of the claimed design.