The ornamental design for a handbag, as shown and described.
FIG. 1 is a perspective view of a handbag;
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 long-short-long broken lines showing a shoulder strap, zippers, a strap attached to a padlock, the padlock, and a buckle strap attached to the handles, form no part of the claimed design. The remaining broken lines represent stitching, which forms no part of the claimed design.