The ornamental design for a handbag, as shown and described in the figures.
FIG. 1 is a front elevation view of a handbag showing my new design;
FIG. 2 is a rear elevation view thereof;
FIG. 3 is left elevation view thereof;
FIG. 4 is a right elevation view thereof;
FIG. 5 is a top, front perspective view thereof;
FIG. 6 is a bottom plan view thereof; and,
FIG. 7 is a front elevation view in an alternate configuration, where the front flap is open.
The dot-dashed lines in the photographs illustrate the boundaries of the claimed design and the portions of the photographs in lighter contrast form no part of the claimed design.