The ornamental design for a pump, as shown and described.
FIG. 1 is a top front perspective view of the pump;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a bottom view thereof;
FIG. 6 is a left side view thereof; and,
FIG. 7 is a right side view thereof.
Light and shade expressed on the surface in each view are for specifying the shape of the surface. The broken lines are included for the purpose of illustrating environmental structure and form no part of the claimed design.