The ornamental design for a keychain, as shown and described.
FIG. 1 is a first perspective view of an embodiment of a key chain, showing the claimed new design;
FIG. 2 is a second perspective view thereof;
FIG. 3 is a front elevation view thereof;
FIG. 4 is a rear elevation view thereof;
FIG. 5 is a left side elevation view thereof;
FIG. 6 is a right side elevation view thereof;
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan view thereof.
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.