The ornamental design for a handbag, as shown and described.
FIG. 1 is a front of the handbag showing my new design;
FIG. 2 is a rear view of the handbag shown in FIG. 1;
FIG. 3 is a left side view of the handbag shown in FIG. 1;
FIG. 4 is a right side view of the handbag shown in FIG. 1; and,
FIG. 5 is a front perspective view of the handbag shown in FIG. 1.
The broken line showing of the zippers, zipper pulls, hanging tags, and rectangular rings is for illustrative purposes only and forms no part of the claimed design.