Plan B;-
Arduino TX to Pi RX use 3 silicon diodes in series (5-3*0.6=3.2V) voltage drop or one 1.8 V Zener diode, either will drop the voltage to Pi levels. Pi TX to Arduino RX just connected, 3.3 volt won't hurt the Arduino and its close enough for the Arduino to receive it.
Use USB or powered USB hub is more common since this time not only Arduino is powered by Pi but also supported up to 127 Arduinos same time.