The ornamental design for an armchair, as shown and described.
FIG. 1 is a front perspective view of a first embodiment of an armchair with a light frame and dark upholstery;
FIG. 2 shows a front elevational view of FIG. 1;
FIG. 3 shows a left side view of FIG. 1, the right side being a mirror image thereof;
FIG. 4 shows a rear side perspective view of FIG. 1;
FIG. 5 shows a rear elevational view of FIG. 1;
FIG. 6 shows a top plan view of FIG. 1;
FIG. 7 shows a bottom plan view of FIG. 1;
FIG. 8 is a front perspective view of second embodiment of an armchair which is identical to FIG. 1, except that it has a dark frame and light upholstery; and,