The ornamental design for the self-leveling paint bucket, as shown and described.
FIG. 1 is a front view of the self-leveling paint bucket.
FIG. 2 is a rear view of the claimed design.
FIG. 3 is a left side view of the claimed design.
FIG. 4 is a top view of the claimed design.
FIG. 5 is a bottom view of the claimed design; and,
FIG. 6 is a perspective view of the claimed design in use on a roof, which is depicted in dashed lines and is environmental matter forming no part of the claimed design.