Python Mock two functions in Pytest test using MagicMock

This is the function I am trying to test.

def service_request(url, session=configure_session()):
    response = session.get(url)

    document = json.loads(response.text)
    return document

Yes it is basic right now (hardly worth testing you might argue), but if I can get the mocking working I will be able to add more logic to it and test the new additions as I go. I have tried various things to mock two other functions called within this test. One is the configure_session function and the other is the get method from the requests library.

The configure_session function builds the requests.Session object configured with certificates required to make a request to the service. This method also optionally verifies the SSL connection with the CA bundle. The config file, imported at the top of the file (not shown) retrieves environment variables and is working as expected.

def configure_session():
    session = requests.Session()

    session.cert = (config.client_cert, config.client_key)
    if config.ssl_verify:
        session.verify = config.ca_bundle
    else:
        session.verify = False

    user_agent = f'{config.user_agent_version}'
    session.headers.update({'User-Agent': user_agent,
                     'Accept': 'application/json'})
    return session

This is the latest code I have tried. The load_fixture method is a test utility method and is working as expected.

def test_request():
    response_fixture = load_fixture('example_response.json')

    mock_session = MagicMock()
    mock_get = MagicMock()
    mock_session.get.return_value = mock_get
    mock_get.text.return_value = response_fixture

    test_url = "https://url-for-test/with/the/path/here"

    expected_headers = { 
        'Accept': 'application/json',
        'User-Agent': 'test_user_agent_from_env_vars'
         }

    service_request(test_url, mock_session)

    mock_session.get.assert_called_with(
        test_url,
        headers=expected_headers
        )
    assert response == response_fixture

The current error is TypeError: the JSON object must be str, bytes or bytearray, not 'MagicMock'.