The ornamental design for a stylet, as shown and described.
FIG. 1 is a perspective view of a stylet showing my new design;
FIG. 2 is a top view thereof;
FIG. 3 is a front view thereof; and,
FIG. 4 is a bottom view thereof.
The broken lines showng a plunger is for illustrative purposes and forms no part of the claimed design.