The ornamental design for a stool, as shown and described.
FIG. 1 is a perspective view of a stool showing the new design;
FIG. 2 is a right side view, the left side view being a mirror thereof;
FIG. 3 is a front view;
FIG. 4 is a rear view;
FIG. 5 is a top view; and,
FIG. 6 is a bottom view.
The broken lines shown in the drawings are for the purpose of illustrating portions of the article and form no part of the claimed design. The surface shading is intended to depict surface contour and forms no part of the claimed design.