The ornamental design for a stylus, as shown and described.
FIG. 1 is a perspective view of a stylus showing my new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a left side elevation view thereof;
FIG. 5 is a right side elevation view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is perspective view thereof, showing the stylus in a position of use shown with environment, the broken lines are for purpose of illustrating environmental structure and forms no part of the claimed design.