The ornamental design for a stepframe, as shown and described.
FIG. 1 is a front perspective view of the step frame of the present invention;
FIG. 2 is a rear view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a left-side view thereof;
FIG. 5 is a right-side view thereof;
FIG. 8 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The dot-dashed broken lines shown in the figures are for the purpose of illustrating boundaries of the claimed design and form no part of the claimed design.