I claim the ornamental design for a single lever handle, as shown and described.
FIG. 1 shows the front perspective view of a single lever handle showing my new design;
FIG. 2 shows the elevated right side view thereof, with left sides being the mirror image of;
FIG. 3 shows the top view thereof;
FIG. 4 is a bottom view thereof; (the broken lines shown in the drawings are for illustrative purposes only and form no part of the claimed design); and,