How to apply a decorator to a MagicMock function in unittest?

Question

I am writing a test for a library using unittest. Real usage of the library would look like:

from mylibrary import Connection

con = Connection()

@con.notification(int)
def my_callback(value):
    print('New value:', value)

con.add_notification(my_callback)

# A remote change will now trigger the callback

Now I want to mock the callback in a unit test so I can verify it got called with the expected arguments:

import unittest
from unittest import mock

class LibraryTestCase(unittest.TestCase):

    # ...

    def my_test(self):
        mock_callback = mock.MagicMock()
        self.con.add_notification(mock_callback)
        self.con.write(3.14)  # Trigger change

        mock_callback.assert_called_once()  # This succeeds

This verifies the callback got triggered, but the callback was not decorated (this is needed to specify the variable type). How can I decorate a mocked object?
Pseudo code to illustrate what I am looking for: (not related to decorated variables)

        @con.notification(int)
        mock_callback = mock.MagicMock() 

Let me emphasize the order: I want to use a real, unmodified decorator with a mock function.