The ornamental design for string lights, as shown and described.
FIG. 1 is a perspective view of string lights showing my new design;
FIG. 2 is another perspective view thereof;
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 string lights are shown with a symbolic break in its length and the appearance of any portion of the string lights between the break lines form no part of the claimed design. The broken lines in the drawings depict portions of the string lights that form no part of the claimed design.