The ornamental design for a pour spout for a bucket, as shown and described.
FIG. 1 is a perspective view of a pour spout for a bucket showing my new design as attached to a bucket; it being understood that the broken lines are for illustrative purposes only and form no part of the claimed design.
FIG. 2 is an outer side view of the design of the pour spout for a bucket.
FIG. 3 is an inner side view.
FIG. 4 is a left side view; the right side being a mirror image.