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; and,
FIG. 6 is a top view in an alternate configuration, where the handbag is open.
The broken lines of uneven length define the boundary of the interior of the handbag and form no part of the claimed design. The broken lines of uniform length shown in all views represent stitching, which is part of the claimed design.