The ornamental design for a handbag, as shown and described.
FIG. 1 is a front elevational view of a handbag showing my new design;
FIG. 2 is a rear elevational view thereof;
FIG. 3 is a left side perspective view thereof;
FIG. 4 is a right side perspective view thereof;
FIG. 5 is a top plan view thereof; and,
FIG. 6 is a bottom view thereof.
The broken lines showing a handbag are for illustrative purposes and form no part of the claimed design. The broken lines shown within the full line disclosure portions of the claimed design are understood to represent stitching and do not form part of the claimed design.