The ornamental design for a handbag, as shown and described.
FIG. 1 is a front perspective 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 front view thereof;
FIG. 5 is a back view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken line showing of portions of the body of the bag, zipper, zipper pull, zipper track, rivets, and handles illustrate portions of the article that form no part of the claimed design.