The ornamental design for a pillowcase, as shown and described.
FIG. 1 is a front perspective view of a pillowcase, showing my new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a bottom plan view thereof;
FIG. 4 is a front elevation view thereof;
FIG. 5 is a rear elevation view thereof;
FIG. 6 is a left-side elevation view thereof; and,
FIG. 7 is a right-side elevation view thereof.
The evenly-sized dot-dot broken lines depict portions of the pillowcase that form no part of the claimed design. The unevenly-sized dot-dash-dot broken line in FIG. 1 depicts the bounds of the claimed design through forms no part thereof.