The ornamental design for a pipette, as shown and described.
FIG. 1 is a front view of a pipette according to the present design;
FIG. 2 is a rear view;
FIG. 3 is a left side view;
FIG. 4 is a right side view;
FIG. 5 is a rear perspective view; and,
FIG. 6 is a front perspective view.