How to pass Mojo function to Python in Python interop?

The question is how to pass Mojo function to Python in Python interop?

For example,

# This is main.mojo
from python.python import Python

def callback():
    return 5

def main():
  Python.add_to_path(".")
  let test_module = Python.import_module("lib")
  print(test_module.test_interop(callback))

# This is lib.py
def test_interop(func):
    return func()

If I run this, it will show the following message:

$ main.mojo
main.mojo:9:33: error: invalid call to '__call__': argument #1 cannot be converted from 'fn() raises -> object' to 'PythonObject'
  print(test_module.test_interop(callback))
        ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~
main.mojo:1:1: note: function declared here
from python.python import Python
^
mojo: error: failed to parse the provided Mojo