I claim the ornamental design for a sticker spool, as shown and described.
FIG. 1 is a perspective view of my new design;
FIG. 2 is a front elevational view thereof; the rear elevational view, right side elevational view and left side elevational view thereof being identical to said front elevational view; and,
FIG. 3 is a top plan view thereof; the bottom plan view thereof being identical to said top plan view.