The ornamental design for a pillowcase, as shown and described.
FIG. 1 is a front elevational view of an embodiment of a pillowcase;
FIG. 2 is a rear elevational view of the pillowcase;
FIG. 3 is a top plan of the pillowcase;
FIG. 4 is a bottom plan view of the pillowcase;
FIG. 5 is a right elevational view of the pillowcase; and,
FIG. 6 is a left elevational view of the pillowcase.
The broken lines shown in FIGS. 1-6 are included for the purpose of illustrating environmental elements only and form no part of the claimed design. The hook and loop fasteners shown in FIG. 3 form no part of the claimed design.