unix/file: Parse "buffering" argument of open() builtin.

It's ignored (unbuffered, raw I/O is used), but least makes it compatible
with CPython.

unix/file.c

@@ -148,6 +148,7 @@ STATIC MP_DEFINE_CONST_FUN_OBJ_1(fdfile_fileno_obj, fdfile_fileno);
 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)} },
 };
 #define FILE_OPEN_NUM_ARGS MP_ARRAY_SIZE(file_open_args)

unix/qstrdefsport.h

@@ -30,6 +30,7 @@ Q(Test)
 
 Q(fileno)
 Q(makefile)
+Q(buffering)
 
 Q(FileIO)
 Q(flush)