The ornamental design for a handbag, as shown and described.
FIG. 1 is a front perspective view of the handbag.
FIG. 2 is a left side view of the handbag.
FIG. 3 is a front view of the handbag.
FIG. 4 is a right side view of the handbag.
FIG. 5 is a top view of the handbag; and,
FIG. 6 is a rear view of the handbag.
The broken lines showing a handbag are for illustrative purposes only and form no part of the claimed design. The broken lines around the perimeter of the handbag handle represent stitching and is part of the claimed design.