I claim the ornamental design for an above-ground swimming pool, as shown and described.
FIG. 1 is a side view showing a portion of a continuous side wall between three vertical pillars showing my new design.
FIG. 2 is a top view of my new design; and,
FIG. 3 is a perspective view of my new design showing the entire above-ground swimming pool and the repeating pattern of the side wall panels and vertical pillars.