conftest.py

@pytest.fixture
def mock_utcnow(monkeypatch):

    def wrapped(now=datetime.min):
        monkeypatch.setattr(utils, "get_utcnow", lambda: now)

    return wrapped

test.py

from datetime import datetime

def test(mock_utcnow):
    fake_now = datetime.utcnow()
    mock_utcnow(fake_now)
    another_fake_now = datetime.utcnow()
    mock_utcnow(fake_now)