The ornamental design for a skateboardbackpack, as shown and described.
FIG. 1 is a left side elevational view of a skateboard backpack showing my new design;
FIG. 2 is a right side elevational view thereof;
FIG. 3 is a bottom plan view thereof;
FIG. 4 is a top plan view thereof;
FIG. 5 is a bottom perspective view thereof; and,
FIG. 6 is a top plan view shown in an opened condition.
The structure depicted by broken lines in FIGS. 4 through 6 has been shown for the purpose of illustrating environment that forms to part of the claimed design.