The ornamental design for a pennib, as shown and described.
FIG. 1 is a perspective view of a pen nib;
FIG. 2 is a front view of a pen nib;
FIG. 3 is a rear view of a pen nib;
FIG. 4 is a left side elevation of a pen nib;
FIG. 5 is a top plan view of a pen nib; and,
FIG. 6 is a bottom view of a pen nib.
The broken lines showing the feature at the end in FIGS. 2, 3, 5 and 6 are included for the purpose of showing a portion of the pen nib that forms no part of the claimed design.