The ornamental design for a tube, as shown and described.
FIG. 1 is a perspective view of a tube showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The tube is shown with a symbolic break in length. The dot-dash-jagged broken lines, the brackets and the appearance of any portion between the break lines form no part of the claimed design.