extmod/moduos_dupterm: Reserve buffer bytearray object for dupterm.

Allocating it for each read/write operation is a memory fragmentation
hazard.
diff --git a/py/mpstate.h b/py/mpstate.h
index b79fd7c..2817957 100644
--- a/py/mpstate.h
+++ b/py/mpstate.h
@@ -143,6 +143,7 @@
 
     #if MICROPY_PY_OS_DUPTERM
     mp_obj_t term_obj;
+    mp_obj_t dupterm_arr_obj;
     #endif
 
     #if MICROPY_PY_LWIP_SLIP