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 of a handbag;
FIG. 3 is a rear view of a handbag;
FIG. 4 is a left side view of a handbag;
FIG. 5 is a right side view of a handbag;
FIG. 6 is a top view of a handbag; and,
FIG. 7 is a bottom view of a handbag.
The broken lines shown in the figures are included for the purpose of illustrating the environment in which the design is used and form no part of the claimed design. The dashed lines on the surface of the claimed design represent unclaimed stitching.