The ornamental design for a handbag, as shown and described.
1.1 is a front view of a handbag;
1.2 is a rear view thereof;
1.3 is a left side view thereof;
1.4 is a right side view thereof;
1.5 is a bottom view thereof;
1.6 is a top view thereof; and
1.7 is a perspective view thereof.
The two small circular elements shown in broken lines on the left and right side of 1.2 and the closure elements of the circular structures connecting the handle to the bag shown in broken lines in 1.1-1.4 and 1.7 form no part of the claimed design. All other broken lines indicate stitching which does form a part of the claimed design.