The ornamental design for a tow tractor, as shown and described.
FIG. 1 is a front, top and left side perspective view of a tow tractor showing our new design;
FIG. 2 is a rear, top and right side perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a right side view thereof; and,
FIG. 7 is a left side view thereof.
The broken lines illustrate portions of the tow tractor that form no part of the claimed design. The oblique shade lines shown in the drawings indicate a transparent surface. The bottom plan view is omitted since it is unornamented and forms no part of the claimed design.