The ornamental design for a pair of slippers, as shown and described.
FIG. 1 is a left side perspective view of a pair of slippers showing my new design;
FIG. 2 is a right side perspective view thereof; and,
FIG. 3 is a top perspective view thereof.
Portions of FIGS. 1 through 3 are shown in evenly spaced broken lines illustrating portions of the article that form no part of the claimed design. The dash-dot-dash boundary lines are directed to unclaimed features and form no part of the claimed design.