The ornamental design for a penshell, as shown and described.
FIG. 1 is a perspective view of pen shell showing my new design.
FIG. 2 is a front view thereof.
FIG. 3 is a rear view thereof.
FIG. 4 is a right view thereof.
FIG. 5 is a left view thereof.
FIG. 6 is a top view thereof; and,
FIG. 7 is an enlarged bottom view thereof.
The broken lines in the drawings are included for the purpose of illustrating environmental detail and form no part of the claimed design, except for the broken line oval shown in the aperture of the pen shell in FIG. 1 and the broken line circle immediately inside the wall of the pen shell shown in FIG. 6, which define the bounds of the claimed design, forming no part thereof.