The ornamental design for a dropbox, as shown and described.
FIG. 1 is a front, left side perspective view of a drop box showing our new design, with the front, right side perspective view being a mirror image thereof;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a left side elevation view thereof, with the right side elevation view being a mirror image thereof;
FIG. 4 is a rear elevation view thereof; and,
FIG. 5 is a top plan view thereof.
The bottom view forms no part of the claimed invention. The broken lines of even length shown in the drawings illustrate portions of the drop box that form no part of the claimed design.