The ornamental design for a handbag, as shown and described.
FIG. 1 is a front elevation view of a handbag showing my new design;
FIG. 2 is a right side view thereof;
FIG. 3 is a left side view thereof;
FIG. 4 is a rear elevation view thereof;
FIG. 5 is a bottom view thereof;
FIG. 6 is a front view in an alternate configuration, where the handbag is open; and,
FIG. 7 is a top view thereof.
The dotted lines on the photograph showing the outline of the bag interior are for the purposes of illustrating environment and form no part of the claimed design.