The ornamental design for a stair tread, as shown and described.
FIG. 1 is a perspective view for a design for a stair tread according to the invention;
FIG. 2 is a front elevational view for the design for a stair tread; and,
FIG. 3 is a top plan view for the design for a stair tread.
The figures illustrate a stair tread in which the forward portion of the stair tread and the remainder of the stair tread have a visually contrasting appearance as shown by the line shading on the forward portion of the stair tread and the stippling on the remainder of the stair tread.