The ornamental design for a pivot pin, as shown and described.
FIG. 1 is a perspective view of a pivot pin showing my new design;
FIG. 2 is a side view thereof;
FIG. 3 is a rear end view thereof;
FIG. 4 is a front end view thereof; and,
FIG. 5 is a perspective view showing the pivot pin positioned with respect to a trailer pivot pin receiver shown in broken lines.
The dot-dash-dot broken lines rendered in the drawings illustrate portions of the pivot pin and form no part of the claimed design. The broken lines showing of the trailer in FIG. 5 illustrates the environment of the claimed design and forms no part thereof.