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 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 view thereof; and,
FIG. 7 is a bottom view thereof.
The portions depicted in broken lines are environment and form no part of the claimed design. The broken line shown below the zipper in FIG. 3 represents stitching and forms no part of the claimed design.