The ornamental design for a pillowcase, as shown and described.
FIG. 1 is a perspective view of the pillowcase;
FIG. 2 is a front elevational view of the pillowcase;
FIG. 3 is another front elevational view of the pillowcase;
FIG. 4 is a rear elevational view of the pillowcase;
FIG. 5 is bottom plan view of the pillowcase;
FIG. 6 is a top plan view of the pillowcase;
FIG. 7 is a left side elevational view of the pillowcase; and,
FIG. 8 is a right side elevational view of the pillowcase.
The broken lines shown in FIGS. 1-8 are included for the purpose of illustrating environmental elements only and form no part of the claimed design. The handle, pockets, bottom of the pillowcase, strap of pillow toy, and pillow toy form no part of the claimed design.