The ornamental design for a handbag, as shown and described.
FIG. 1 is a front perspective view of the handbag according to the 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 on the periphery and surface of the handbag form no part of the claimed design. The broken lines on the handles of the handbag represent stitching and form a part of the claimed design.