unix microsocket: Add dummy makefile() method.

Unlike CPython socket, microsocket object already implements stream protocol
(read/write methods), so makefile() just returns object itself. TODO: this
doesn't take care of arguments CPython's makefile() may accept.

py/obj.c

@@ -253,3 +253,4 @@ mp_obj_t mp_obj_len_maybe(mp_obj_t o_in) {
 mp_obj_t mp_identity(mp_obj_t self) {
     return self;
 }
+MP_DEFINE_CONST_FUN_OBJ_1(mp_identity_obj, mp_identity);

py/obj.h

@@ -370,6 +370,7 @@ extern const mp_obj_type_t fun_bc_type;
 void mp_obj_fun_bc_get(mp_obj_t self_in, int *n_args, uint *n_state, const byte **code);
 
 mp_obj_t mp_identity(mp_obj_t self);
+MP_DECLARE_CONST_FUN_OBJ(mp_identity_obj);
 
 // super
 extern const mp_obj_type_t super_type;

unix/socket.c

@@ -216,6 +216,7 @@ static mp_obj_t socket_make_new(mp_obj_t type_in, uint n_args, uint n_kw, const
 
 static const mp_method_t microsocket_type_methods[] = {
         { "fileno", &socket_fileno_obj },
+        { "makefile", &mp_identity_obj },
         { "read", &mp_stream_read_obj },
         { "readall", &mp_stream_readall_obj },
         { "readline", &mp_stream_unbuffered_readline_obj},