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 left side view thereof, the right side view is a mirror image of the left side view;
FIG. 5 is a top view thereof;
FIG. 6 is a bottom view thereof; and,
FIG. 7 is an enlarged view at a portion indicated by a line of the reference number 7 in FIG. 1.
The broken line showing of the piano is included for the purpose of illustrating unclaimed portions of the piano and forms no part of the claimed design.