The ornamental design for a lightbulb, as shown and described.
FIG. 1 is a first perspective view of an embodiment of a light bulb, showing the claimed new design;
FIG. 2 is a second perspective view of an embodiment of a light bulb;
FIG. 3 is a front elevation view of the light bulb shown in FIG. 1;
FIG. 4 is a rear elevation view of the light bulb shown in FIG. 1;
FIG. 5 is a left side elevation view of the light bulb shown in FIG. 1;
FIG. 6 is a right side elevation view of the light bulb shown in FIG. 1;
FIG. 7 is a top plan view of the light bulb shown in FIG. 1; and,
FIG. 8 is a bottom plan view of the light bulb shown in FIG. 1.
The broken lines in the drawings represent boundaries of the claimed design. The broken lines are environmental structures that form no part of the claimed design. The broken lines and unshaded regions bounded by broken lines form no part of the claimed design.