The ornamental design for a magnetic keyholder, as shown and described.
FIG. 1 is a front perspective view of a magnetic key holder showing my new design, wherein a wallet and key fob are included to demonstrate environment;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a left side elevation view thereof;
FIG. 4 is a right side elevation view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a bottom plan view thereof; and,
FIG. 7 is an enlarged, rear elevation view of the magnetic key holder.
The broken lines shown in the figures represent a wallet and a key fob and form no part of the claimed design. The broken lines shown in FIG. 7 represent magnets and an adhesive strip and form no part of the claimed design.