Python: Add setuptools support for bluetooth_packets_python3

* Add cert/bluetooth_packets_python3_setup.py just in case a test
  runner wants to compile the source against their own non-Python3.8
  interpreter

Bug: 143374372
Test: make and import bluetooth_packets_pyton3
Change-Id: I01330fcf0c9e98ae178507ac775d817d1ef82133