The ornamental design for a sleeve for a waterpitcher, as shown and described.
FIG. 1 is a front perspective view of the sleeve of the present invention;
FIG. 2 is a top view thereof;
FIG. 3 is a bottom view thereof;
FIG. 4 is a left side view thereof, the right side being a mirror image;
FIG. 5 is a front side view thereof; and,
FIG. 6 is a back side view thereof.
The broken line showing of environmental structure is included for the purpose of illustrating portions of the “article” and forms no part of the claimed design.