Loky backend sometimes randomly mixes items

This test randomly fail but ideally it shouldn't
```python
from dpipe.batch_iter import Loky
from dpipe.batch_iter.pipeline import wrap_pipeline

size = 100
    for i, item in enumerate(wrap_pipeline(range(size), Loky(lambda x: x ** 2, n_workers=2))):
        assert item == i ** 2
```