Bi-Directional Logic Level Converter
I want to use a written program for ATtiny that only generates a TX UART output on the software side and send data to the raspberry.
I need 9600 baud

In this situation a resistive voltage divider should work fine; in fact with decently chosen resistors (say 1.2K series and 2.2K shunt) it will deliver a better signal than most off-the-shelf implementations of a series MOSFET shifter which tend to use a 10K pullup resistor.
Another option could be to run your ATtiny at 3v3 itself; however if you do that it is better to give it its own 5v to 3v3 voltage regulator, as even inadvertently connecting or disconnecting loads (especially anything that may have its own supply filter capacitor) from the pi's delicate 3v3 pin tends to cause system failures.