Monkeypatch imported module from other file with pytest

As the title says. I want to mock an imported function from a module. In this case the module is datetime and I want to mock datetime.datetime.now. I included what I've done without success.

# main.py:
import datetime


# cannot modify
def call_me(func):
    return func()


class A:
    variable = call_me(datetime.datetime.now)

# conftest.py:
import pytest
import datetime

FAKE_TIME = datetime.datetime.fromisoformat("2020-03-19T03:30:00")


@pytest.fixture(autouse=True)
def patch_datetime_now(monkeypatch):
    class mydatetime(datetime.datetime):
        @classmethod
        def now(cls):
            return FAKE_TIME

    monkeypatch.setattr('datetime.datetime', mydatetime)

# test_main.py:
import datetime
from main import A


def test_main():
    assert A.variable == datetime.datetime.now()

❯ pytest
------------------------------------------------------------------------------------
def test_main():
>       assert A.variable == datetime.datetime.now()
E       AssertionError: assert datetime.datetime(2020, 3, 19, 21, 32, 39, 861956) == datetime.datetime(2020, 3, 19, 3, 30)

I searched for a workaround for this and only found this question How to monkeypatch python's datetime.datetime.now with py.test?. The code included is just a minimal example of what I need. Basically I have an app who uses sqlalchemy.orm for saving data and a specific model have a default value for the date equal to datetime.datetime.now and I need to modify the date of the default value to be able to test it. The models are defined in other file.

Thank you for your help.