The ornamental design for a piano, as shown and described.
FIG. 1 is a perspective view of the front, top and right side of a piano showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a top plan view thereof;
FIG. 5 is a bottom view thereof;
FIG. 6 is a left side view thereof;
FIG. 7 is a left side view thereof; and,
FIG. 8 is an enlarged view at 8-8 line thereof in FIG. 7.
The broken lines immediately adjacent the shaded areas represent the bounds of the claimed design while all other broken lines are directed to environment and are for environmental purposes only; the broken lines form no part of the claimed design.