py, unix: Allow to compile with -Wunused-parameter.

See issue #699.

py/sequence.c

@@ -98,6 +98,8 @@ bool mp_seq_get_fast_slice_indexes(mp_uint_t len, mp_obj_t slice, mp_bound_slice
 #endif
 
 mp_obj_t mp_seq_extract_slice(mp_uint_t len, const mp_obj_t *seq, mp_bound_slice_t *indexes) {
+    (void)len; // TODO can we remove len from the arg list?
+
     mp_int_t start = indexes->start, stop = indexes->stop;
     mp_int_t step = indexes->step;