We claim the ornamental design for a sheath, as shown and described.
FIG. 1 is a perspective view of the sheath showing our new design;
FIG. 2 is a perspective view of the sheath shown in FIG. 1 with the environment removed;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof;
FIG. 5 is a front view thereof;
FIG. 6 is a rear view thereof;
FIG. 7 is a right side view thereof; and,
FIG. 8 is a left side view thereof.
The ornamental design which is claimed is shown in solid lines in the drawings. The broken lines in the drawings are for illustrative purposes only and form no part of the claimed design.