The ornamental design for a lantern, as shown and described.
FIG. 1 is a left side elevational view of a lantern comprising the new design;
FIG. 2 is a front elevational view;
FIG. 3 is a right side elevational view;
FIG. 4 is a rear elevational view;
FIG. 5 is a top view; and,
FIG. 6 is a front perspective view.
The bottom is flat and devoid of surface ornamentation. The broken line showing represents portions of the article not claimed only and forms no part of the claimed design.