Add alternate name VOLTAGE_MONITOR for A3

Author: xorbit

ports/raspberrypi/boards/raspberry_pi_pico/pins.c

@@ -36,5 +36,6 @@ STATIC const mp_rom_map_elem_t board_global_dict_table[] = {
     { MP_ROM_QSTR(MP_QSTR_GP28), MP_ROM_PTR(&pin_GPIO28) },
     { MP_ROM_QSTR(MP_QSTR_A2), MP_ROM_PTR(&pin_GPIO28) },
     { MP_ROM_QSTR(MP_QSTR_A3), MP_ROM_PTR(&pin_GPIO29) },
+    { MP_ROM_QSTR(MP_QSTR_VOLTAGE_MONITOR), MP_ROM_PTR(&pin_GPIO29) },
 };
 MP_DEFINE_CONST_DICT(board_module_globals, board_global_dict_table);