The ornamental design for a spirometer, as shown and described.
FIG. 1 is a front perspective view of a spirometer, 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;
FIG. 7 is a bottom view thereof;
FIG. 8 is a bottom perspective view thereof;
FIG. 9 is a front perspective view thereof, shown in a closed state; and,
FIG. 10 is a front perspective view thereof, shown with environment.
The broken lines immediately adjacent to the shaded areas depict the bounds of the claimed design and form no part of the claimed design, while all other broken lines depict environment and form no part of the claimed design.