The ornamental design for a satchel, as shown and described.
FIG. 1 is a perspective view of a satchel showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a back view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken lines in FIGS. 1-6 showing a tag attached to the satchel depict environment and form no part of the claimed design. The broken lines on the satchel illustrate stitching and form no part of the claim.