The ornamental design for a handbag, as shown and described.
FIG. 1 is a front elevation view of a handbag showing my new design;
FIG. 2 is a top and end perspective view thereof; and,
FIG. 3 is a top and end perspective view thereof illustrating the handbag in an open position.
The dashed lines represent stitching that forms a part of the claimed design. The broken lines in FIG. 3 are for illustrative purposes only and form no part of the claimed design.