The ornamental design for a penholder, as shown and described.
FIG. 1 is a perspective view of a pen holder showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof;
FIG. 8 is another perspective view thereof; and,
FIG. 9 is a perspective view thereof showing a pen inserted in the pen holder.
The pen in broken lines in FIG. 9 is included for the purpose of illustrating the environment of the pen holder and forms no part of the claimed design. The broken lines on the bottom of the pen holder shown in FIG. 7 are included to show portions of the pen holder that form no part of the claimed design.