The ornamental design for a pantstrap, as shown and described.
FIG. 1 is an isometric view of the front of the pant strap, which embodies my design;
FIG. 2 is a back view of the pant strap;
FIG. 3 is a side view of the pant strap;
FIG. 4 is a front view of the pant strap;
FIG. 5 is an illustrative view of the pant strap showing my new design in a position of use with the pants rolled down. The broken line showing of the pants and foot in FIG. 5 depicts unclaimed environment only and forms no part of the claimed design; and,
FIG. 6 is an illustrative view of the pant strap showing my new design in a position of use. The broken line showing of the pants and foot in FIG. 6 depicts unclaimed environment only and forms no part of the claimed design.