We claim the ornamental design for an air pump cart, as shown and described.
FIG. 1 is a front perspective view showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The equal length broken lines in the figures show portions of the air pump cart, which form no part of the darned design. The dash-dot broken lines define bounds of the claimed design and form no part of the claimed design.