FIG. 1 is a perspective view of a bottle opener; FIG. 2 is a front view thereof; FIG. 3 is a back 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; and, FIG. 8 is a second perspective view thereof, with environmental subject matter shown in broken line and forming no part of the claimed design.