We claim the ornamental design for a shopping cart, as shown and described.
FIG. 1 is a perspective view of a shopping cart showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a back view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof; and,
FIG. 6 is a top view thereof.
The bottom views are not part of the claimed design. The elements shown in broken lines are included for the purpose of illustrating portions of the shopping cart that form no part of the claimed design.