The ornamental design for a painter's hook, as shown and described.
FIG. 1 is a perspective view of a painter's hook, showing my new design;
FIG. 2 is a right side elevational view thereof;
FIG. 3 is a left side elevational view thereof;
FIG. 4 is a back side elevational view thereof;
FIG. 5 is a front side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines shown in the drawings represent portions of the painter's hook that form no part of the claimed design. The dot-dash line(s) which define the bounds of the claimed design form no part thereof.