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 elevational view thereof;
FIG. 4 is a right side elevational view thereof; and,
FIG. 5 is a bottom plan view thereof.
The claimed design is shown in the figures by solid lines as well as by dashed lines that represent stitching. The doted lines shown in the FIG. 2 are for illustrative purposes only and form no part of the claimed design.