The ornamental design for a handbag with a butterfly, as shown and described.
FIG. 1 is a perspective front view of a handbag showing my new design;
FIG. 2 is a front view of the handbag shown in FIG. 1;
FIG. 3 is a rear view of the handbag shown in FIG. 1;
FIG. 4 is a left side view of the handbag shown in FIG. 1; and,
FIG. 5 is a top view of the handbag shown in FIG. 1.
The broken lines showing a handbag and interior panels of the butterfly are for the purposes of illustrating environment and form no part of the claimed design.