The ornamental design for a piece of luggage, as shown and described.
FIG. 1 is a perspective view of the piece of luggage, illustrating our new design;
FIG. 2 is an elevational view of one side of the piece of luggage;
FIG. 3 is an elevational view of the other side of the piece of luggage;
FIG. 4 is a front elevational view of the piece of luggage;
FIG. 5 is a rear elevational view of the piece of luggage;
FIG. 6 is a top plan view of the piece of luggage; and,
FIG. 7 is a bottom plan view of the piece of luggage.
The broken lines immediately adjacent the shaded areas represent the bounds of the claimed design, while all other broken lines are directed to environment and are for illustrative purposes only; the broken lines form no part of the claimed design.