The ornamental design for a pump, as shown and described.
FIG. 1 is a top right front perspective view of a pump according to my design in which broken lines show environmental structure that is not part of the claimed design;
FIG. 2 is a front bottom right perspective view thereof;
FIG. 3 is a bottom planned view thereof;
FIG. 4 is a right side elevational view thereof, the left side elevational view being a mirror image thereof;