The ornamental design for a piano, as shown and described.
FIG. 1 is a front and top perspective view of a piano, showing my new design;
FIG. 2 is a rear and bottom perspective view thereof;
FIG. 3 is a front elevation view thereof;
FIG. 4 is a rear elevation view thereof;
FIG. 5 is a left side elevation view thereof;
FIG. 6 is a right side elevation view thereof;
FIG. 7 is a top plan view thereof;
FIG. 8 is a bottom plan view thereof; and,
FIG. 9 is another perspective view thereof, shown in a using position.
The dashed lines in the figures illustrate portions of the piano that form no part of the claimed design. The evenly dashed lines showing of the piano keys in FIG. 9 depict environmental structure that form no part of the claimed design.