all: Add missing imports for micropython.const.

Found by Ruff checking F821.
Signed-off-by: NAngus Gratton <angus@redyak.com.au>

examples/bluetooth/ble_uart_repl.py

@@ -7,6 +7,7 @@ import bluetooth
 import io
 import os
 import micropython
+from micropython import const
 import machine
 
 from ble_uart_peripheral import BLEUART

ports/nrf/examples/ssd1306_mod.py

@@ -19,6 +19,7 @@
 # disp = SSD1306_I2C_Mod(128, 64, i2c)
 
 from ssd1306 import SSD1306_I2C
+from micropython import const
 
 SET_COL_ADDR = const(0x21)
 SET_PAGE_ADDR = const(0x22)

ports/nrf/examples/ubluepy_eddystone.py

+from micropython import const
 from ubluepy import Peripheral, constants
 
 BLE_GAP_ADV_FLAG_LE_GENERAL_DISC_MODE = const(0x02)

ports/stm32/boards/NUCLEO_WB55/rfcore_debug.py

@@ -35,6 +35,7 @@
 # See rfcore_firmware.py for more information.
 
 from machine import mem8, mem16, mem32
+from micropython import const
 import stm
 
 SRAM2A_BASE = const(0x2003_0000)