The ornamental design for stool, as shown and described.
1.1 is a front perspective view of a stool;
1.2 is a top view thereof;
1.3 is a front view thereof;
1.4 is a side view thereof;
1.5 is a bottom view thereof;
1.6 is a side view thereof, shown stacked; and,
1.7 is an alternate perspective view thereof.
In the drawings, the broken lines showing additional stools illustrate the environment in which the stool is used and forms no part of the claimed design.