The ornamental design for a cupholder, as shown and described.
FIG. 1 shows a perspective view of a cup holder showing my new design suspended from a rail of a shopping cart, stroller or similar apparatus with a cup inserted into and being held by the cup holder; and
FIG. 2 shows a front view of a cup holder showing my new design,
FIG. 3 shows a rear view of a cup holder showing my new design,
FIG. 4 shows a side view of a cup holder showing my new design,
FIG. 5 shows a top view of a cup holder showing my new design; and,
FIG. 6 shows a bottom view of a cup holder showing my new design, wherein the broken lines in FIG. 1 that illustrate the rail and cup are for illustrative purposes only and form no part of the claimed design.