The ornamental design for a pet harness, as shown and described.
FIG. 1 is a perspective view of the pet harness showing my new design;
FIG. 2 is a perspective view thereof from another angle;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a right side elevational view thereof;
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan view thereof.
The broken lines shown in the drawings are included for the purpose of illustrating portions of the pet harness that form no part of the claimed design.