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 elevation view thereof;
FIG. 3 is a rear elevation 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 in the drawings represent stitching and form part of the claimed design. The interior of the handbag is not shown and forms no part of the claimed design.