The ornamental design for a stool, as shown and described.
FIG. 1 is a perspective view of a stool, showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left- side view thereof;
FIG. 5 is a right-side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The equally dashed broken lines in FIGS. 1-3 and 5-7 depict environmental subject matter only and form no part of the claimed design. The dot dash broken line in FIGS. 1-3 and 6-7 is a boundary line representing the bounds of the claimed portion of the design.