obj = bpy.context.object
me = obj.data
bm = bmesh.from_edit_mesh(me)

selected = [e for e in bm.edges if e.select]

seamed_edges = [e for e in selected if e.seam]

if seamed_edges:
    for edge in seamed_edges:
        edge.seam = False
else:
    for edge in selected:
        edge.seam = True

bmesh.update_edit_mesh(me, True)