I claim the ornamental design for a landscape lightpost, as shown and described.
FIG. 1 is a bottom perspective view of a landscape lightpost according to the invention.
FIG. 2 is a front perspective view of a landscape lightpost according to the invention.
FIG. 3 is a side perspective view of a landscape lightpost according to the invention; and,
FIG. 4 is a top perspective view of a landscape lightpost according to the invention.
The broken line showing environmental structures and boundaries are included for the purpose of illustrating only and form no part of the claimed design.