The ornamental design for a handbag, as shown and described.
FIG. 1 is a front, perspective view of a handbag showing my new design;
FIG. 2 is a front view of the thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view.
The broken line showing of the shoulder strap, snaps/buttons, handle, portions of the clasp, and portions of the handbag in the drawings shows unclaimed subject matter.