The ornamental design for a handbag, as shown and described.
2.1 is a front view of a handbag showing my new design;
2.2 is a rear view thereof;
2.3 is a left, front perspective view thereof;
2.4 is a right, rear perspective view thereof;
2.5 is a right side view thereof;
2.6 is a left view thereof;
2.7 is a top view thereof; and
2.8 is a bottom view thereof.
The broken line showing a central circular portion on the rear of the handbag in figures 2.2 and 2.4-2.6 is for the purpose of depicting a portion of the handbag and forms no part of the claim. The evenly dashed broken lines on the edges of claimed surfaces in figures 2.1-2.8 are for the purpose of depicting claimed stitching.