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