The ornamental design for a mop, as shown and described.
FIG. 1 is a top, front, right perspective view of a mop showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a right side view thereof;
FIG. 4 is a left side view thereof; and,
FIG. 5 is a bottom view thereof.
The broken lines immediately adjacent to the shaded area represent unclaimed boundaries of the design. The broken lines showing the remainder of the mop are for environmental purposes only and form no part of the claimed design.