How can pipe data in python without cat command? [duplicate]

I have the following working code in first.py:

import os 
os.system('cat python_better_than_java.wav')

which I can execute like this

python first.py |  ffmpeg -y -f wav -i pipe:0 -f mp3 - >hi.mp3

this works.

I want do this in a pythonic way, so I edited first.py to this:

import sys
with open('python_better_than_java.wav', 'rb') as content_file:
    content = content_file.read()
sys.stdout.write(content)

and executed it using:

python first.py | ffmpeg -y -f wav -i pipe:0 -f mp3 - >hi.mp3

but then it produces the following error:

pipe:0: Invalid data found when processing input

How can I reproduce cat in python?