The ornamental design for a pillowcover, as shown and described.
FIG. 1 is a front view of a pillow cover showing my new design in its open position;
FIG. 2 is a front view of a pillow cover in its closed position;
FIG. 3 is a back view of a pillow cover in its open position;
FIG. 4 is a back view of a pillow cover in its closed position;
FIG. 5 is a left side view of a pillow cover in its open position; and,
FIG. 6 is a left side view of a pillow cover in its closed position.
Top, bottom and right side views not shown are mirror images of right side views. Any broken lines illustrative of environmental structure in the drawings are not part of the design sought to be patented.