v1.15.0.b1

• esp32: Store the buffered LoRaWAN MAC commands before entering deep sleep.
• esp32: Add modutimeq module.
• esp32: Use LTE module Airplane mode to save mode when IDLE (FiPy and GPy).
• esp32: Rename uart.tx_done() to uart.wait_tx_done() and correct the behaviour.
• esp32: Fix crash on LoPy4 due to SX1276 sleep function being outside of IRAM.
• esp32: Make sure that static mutexes are allocated from internal memory only.
• esp32: Fix queue module crash and add full() and empty() methods.
• esp32/sigfox: Update modsigfox libraries to support dual core mode (SiPy, FiPy and LoPy4)
• esp32: mods/modutime.c: ticks_add returns with an unsigned int
• esp32: Fix modbt scan response adv data length
• esp32: Add Bluetooth.CHAR_SUBSCRIBE_EVENT

Additional to that there have been a few fixes on the modbus library: pycom/pycom-modbus@3932d5b

To the Deepsleep shield, Pytrack, Pysense calibration functions before deepsleep: pycom/pycom-libraries@1fbc177

To the Pytrack GPS library: pycom/pycom-libraries@a71e986