I'm testing an endpoint which naturally returns a JSON containing the datetime as a string.

I compare the response content in test as such:

```py
assert serializer_instance.data == {
    "created_at": str(model_instance.created_at),
    "updated_at": str(model_instance.updated_at),
}

```

`created_at` and `updated_at` are surely `DateTimeField`s. However, in this case, test fails saying:

```
E         Differing items:
E         {'created_at': '2020-06-24T12:42:03.578207+03:00'} != {'created_at': '2020-06-24 09:42:03.578207+00:00'}
E         {'updated_at': '2020-06-24T12:42:03.578231+03:00'} != {'updated_at': '2020-06-24 09:42:03.578231+00:00'}

```

So `str` uses a different formatting on datetimes. Sure, the test case can be passed successfully using `strftime`, but there should be an internal function that does it easily in either Django or Django Rest Framework and I'd like to learn it.

Thanks in advance.

---

Environment
===========

* Python 3.8.3
* Django 2.2.12
* Django Rest Framework 3.11.0