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 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.