I claim the ornamental design for a plumbing fitting, as shown and described.
FIG. 1 is a front perspective view of a plumbing fitting;
FIG. 2 is a rear perspective view of the plumbing fitting of FIG. 1;
FIG. 3 is a top view of the plumbing fitting of FIG. 1;
FIG. 4 is a bottom view of the plumbing fitting of FIG. 1;
FIG. 5 is a left side view of the plumbing fitting of FIG. 1;
FIG. 6 is a right side view of the plumbing fitting of FIG. 1;
FIG. 7 is a front view of the plumbing fitting of FIG. 1; and,
FIG. 8 is a rear view of the plumbing fitting of FIG. 1.
The broken lines depict portions of the plumbing fitting that are for illustrative purposes only and form no part of the claimed design. The plumbing fitting is shown with a symbolic break in its length. The appearance of any portion of the plumbing fitting between the break lines forms no part of the claimed design.