The ornamental design for a handbag, as shown and described.
FIG. 1 is a bottom perspective view of a handbag showing our new design;
FIG. 2 is a bottom view thereof;
FIG. 3 is a top view thereof;
FIG. 4 is a front view thereof;
FIG. 5 is a rear view thereof;
FIG. 6 is a right side view thereof; and,
FIG. 7 is a left side view thereof.
The broken lines immediately adjacent to the claimed areas represent the bounds of the claimed design and form no part thereof. The broken lines depicting of the remainder of the handbag show features that form no part of the claimed design.