unix/file: "encoding" arg to open() isn't kw-only.

And with "buffering" arg introduced, it's non possible to make it
non-kwonly.

unix/file.c

@@ -149,7 +149,7 @@ STATIC const mp_arg_t file_open_args[] = {
     { MP_QSTR_file, MP_ARG_OBJ | MP_ARG_REQUIRED, {.u_rom_obj = MP_ROM_PTR(&mp_const_none_obj)} },
     { MP_QSTR_mode, MP_ARG_OBJ, {.u_obj = MP_OBJ_NEW_QSTR(MP_QSTR_r)} },
     { MP_QSTR_buffering, MP_ARG_OBJ, {.u_rom_obj = MP_ROM_PTR(&mp_const_none_obj)} },
-    { MP_QSTR_encoding, MP_ARG_OBJ | MP_ARG_KW_ONLY, {.u_rom_obj = MP_ROM_PTR(&mp_const_none_obj)} },
+    { MP_QSTR_encoding, MP_ARG_OBJ, {.u_rom_obj = MP_ROM_PTR(&mp_const_none_obj)} },
 };
 #define FILE_OPEN_NUM_ARGS MP_ARRAY_SIZE(file_open_args)