eevblog-121gw: introduce BLE to UART gateway (EEVBlog 121GW comm helper)

eevblog-121gw: introduce BLE to UART gateway (EEVBlog 121GW comm helper)

Introduce a Python script which relays data which is received via BLE
communication to a UART. This enables immediate use of the serial-dmm
device driver for the EEVBlog 121GW multimeter, until sigrok's serial
communication layer grows (better) support for this physical layer.

Prefer the bluepy(3) Python module over the external gatttool(1) process
since this enables automatic scan for the device's address, and graceful
handling of lost communication, including re-connecting to the device.