The ornamental design for a shackle, as shown and described.
FIG. 1 is a perspective view of a shackle according to a first preferred embodiment of my new design;
FIG. 2 is a second perspective view of the shackle of FIG. 1;
FIG. 3 is a front elevational view of the shackle of FIG. 1;
FIG. 4 is a top plan view of the shackle of FIG. 1;
FIG. 5 is a right side elevational view of the shackle of FIG. 1;
FIG. 6 is a left side elevational view of the shackle of FIG. 1;
FIG. 7 is rear elevational view of the shackle of FIG. 1; and,
FIG. 8 is a bottom plan view of the shackle of FIG. 1.
The dashed lines in FIGS. 1-8 represent environment and are for illustrative purposes only and do not constitute part of the claimed design. The phantom lines in FIGS. 1-8 define the border between claimed and unclaimed matter.