The ornamental design for a handheld shower, as shown and described.
FIG. 1 is a perspective view of a handheld shower showing our new design;
FIG. 2 is a first end view thereof;
FIG. 3 is a second end view thereof;
FIG. 4 is a first side view thereof;
FIG. 5 is a front view thereof; and,
FIG. 6 is a second side view thereof.
The broken line showing on the front and bottom of the claimed design is included for the purpose of illustrating and forms no part of the claimed design.