Support callable functions in lazy_function

[youtux]

This PR adds functionality I'm really missing in this library, the ability to pass callable functions instead of just fixture names.
The change is extremely simple, (took mainly from pytest-factoryboy's LazyFixture, which I'm a maintainer).

For example:

@pytest.fixture
def admin_group():
    return Group.objects.get("admin")

@pytest.fixture
def readonly_group():
    return Group.objects.get("readonly")

@pytest.fixture
def user__groups():
    return []  # empty by default

@pytest.fixture
def user(user__groups):
    return User(groups=user__groups)

@pytest.mark.parametrize(
    "user__groups",
    [lazy_fixture(lambda admin_group, readonly_group: [admin_group, readonly_group])]
)
def test_foo(user, admin_group, readonly_group):
    assert set(user.groups.all()) == {admin_group, readonly_group}

If this PR gets merged, I'm planning of removing pytest-factoryboy's own implementation of LazyFixture, since it would be overcome by this library, which implements it in a much more complete way (only the ability of passing callables was missing for me!)