extmod/vfs: Allow "buffering" and "encoding" args to VFS's open().

These args are currently ignored but are parsed to make it easier to
write portable scripts between CPython and MicroPython.

extmod/vfs.c

@@ -228,11 +228,14 @@ mp_obj_t mp_vfs_umount(mp_obj_t mnt_in) {
 }
 MP_DEFINE_CONST_FUN_OBJ_1(mp_vfs_umount_obj, mp_vfs_umount);
 
+// Note: buffering and encoding args are currently ignored
 mp_obj_t mp_vfs_open(size_t n_args, const mp_obj_t *pos_args, mp_map_t *kw_args) {
     enum { ARG_file, ARG_mode, ARG_encoding };
     static const mp_arg_t allowed_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_rom_obj = MP_ROM_QSTR(MP_QSTR_r)} },
+        { MP_QSTR_buffering, MP_ARG_INT, {.u_int = -1} },
+        { MP_QSTR_encoding, MP_ARG_OBJ, {.u_rom_obj = MP_ROM_PTR(&mp_const_none_obj)} },
     };
 
     // parse args