The ornamental design for a lantern, as shown and described.
FIG. 1 is a perspective view of my lantern:
FIG. 2 is a front view thereof;
FIG. 3 is a left side view thereof, the right side being a mirror image thereof; and,
FIG. 4 is a back view thereof.
The dashed lines in the figures form no part of the claimed design. All other surface(s) or portion(s) of my lantern not shown in the figures or otherwise described in the specification, such as the bottom thereof, form no part of the claimed design.