The ornamental design for a cabin for a tractor, as shown and described.
FIG. 1 is a perspective view of a cabin for a tractor showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a top plan view thereof;
FIG. 5 is a bottom plan view thereof;
FIG. 6 is a left side view thereof; and,
FIG. 7 is a right side view thereof.
The broken line showing in the bottom view of FIG. 5 is included for the purpose of illustrating portions of the cabin for a tractor and forms no part of the claimed design.