The ornamental design for a tube, as shown and described.
FIG. 1 is a front perspective view of my new design;
FIG. 2 is a front elevational view of my new design, the rear elevational side view being identical;
FIG. 3 is a side elevational view of one side of my new design, the opposite elevational side view being identical;
FIG. 4 is a top plan view thereof; and,
FIG. 5 is a bottom plan view thereof.
The broken lines immediately adjacent the shaded areas represent the bounds of the claimed design while all other broken lines are directed to environment and are for illustrative purposes only; the broken lines form no part of the claimed design.