The ornamental design for a skirt, as shown and described.
FIG. 1 is a front view of a skirt showing my new design, where the rear view is identical;
FIG. 2 is a side view thereof and both the right and left side are identical;
FIG. 3 is a top view thereof;
FIG. 4 is an inside out top view thereof;
FIG. 5 is a front view thereof, shown on a person;
FIG. 6 is a side view thereof;
FIG. 7 is a rear view thereof;
FIG. 8 is a front view of the skirt of FIG. 1 worn as a dress as shown on a person, where the bottom of the skirt is flipped upward and the slits at the bottom of the skirt are used as armholes and the waistband rests on the thighs;
FIG. 9 is a side view thereof;
FIG. 10 is a rear view thereof;
FIG. 11 is a front view of the skirt of FIG. 8 worn as a tank top shown on a person with the waistband sitting at the hips;
FIG. 12 is a side view thereof; and,
FIG. 13 is a rear view thereof.
Broken lines as displayed in FIGS. 5-13 are used to display the body of a person and are not part of the claimed design.