I claim the ornamental design for a litter box canopy, as shown and described.
FIG. 1 is a rear perspective view of a litter box canopy, showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a right-side elevational view thereof, the left-side elevational view being a mirror image thereof;
FIG. 5 is a top plan view thereof; and,
FIG. 6 is a bottom plan view thereof.
The broken lines on the pockets of the canopy represent stitching and are part of the claimed design. The broken lines showing a cat litter box in FIG. 1 depict environmental matter and form no part of the claimed design.