The ornamental design for a pillowcase, as shown and described.
FIG. 1 is a front elevational view of the pillowcase;
FIG. 2 is a rear elevational view;
FIG. 3 is a top plan view;
FIG. 4 is a bottom plan view;
FIG. 5 is a right side elevational view; and,
FIG. 6 is a left side elevational view thereof.
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 opening and hook and loop fasteners in FIG. 3 do not form part of the claimed design.