The ornamental design for a mirror for bicyclists, as shown and described.
FIG. 1 is a perspective view of a mirror for bicyclists showing my new design;
FIG. 2 is a rear perspective view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a front elevation view thereof;
FIG. 5 is a side elevation view thereof;
FIG. 6 is an opposite side elevation view thereof;
FIG. 7 is a top view thereof; and,
FIG. 8 is a bottom view thereof.
The broken line outlines of the hat and clip in FIG. 1 depict the environment associated with the claimed design to show the claimed design in use, but forms no part of the claimed design.