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 short evenly dashed broken lines on claimed surfaces depict stitching and form no part of the claimed design. The long evenly dashed broken lines depict portions of the article and form no part of the claimed design.