The ornamental design for a penholder, as shown and described.
FIG. 1 is a top view of a pen holder showing my new design;
FIG. 2 is a front view thereof, the rear view is a mirror image of the front view;
FIG. 3 is a right side view thereof;
FIG. 4 is an isometric view thereof;
FIG. 5 is another isometric view thereof;
FIG. 6 is a bottom view thereof; and,
FIG. 7 is a rear view thereof supporting pens.
The pens and hand in broken lines in FIG. 7 are included for the purpose of illustrating the environment of the pen holder and form no part of the claimed design.