The ornamental design for a handbag, as shown and described.
FIG. 1 is a bottom plan view of my new design, with certain stitching and certain hardware shown in dashed line for illustrative purposes only and forming no part of the claimed design;
FIG. 2 is a front elevation view thereof, the rear elevation view being a mirror image thereof;
FIG. 3 is a right side elevation view thereof, the left side elevation view being a mirror image thereof; and,