The ornamental design for a handbag, as shown and described.
FIG. 1 is a front perspective view of a handbag showing my handbag design;
FIG. 2 is a front view of the handbag;
FIG. 3 is a left side view of the handbag;
FIG. 4 is a right side view of the handbag; and,
FIG. 5 is a top view of the handbag.
The broken line showing of the handles, grommets, straps, buckle, and portions of the bag structure is for illustrative purposes only and forms no part of the claimed design.