Latest dill release raises exception
## Describe the bug
As reported by @sgugger, latest dill release is breaking things with Datasets.

```
______________ ExamplesTests.test_run_speech_recognition_seq2seq _______________


self = <multiprocess.pool.ApplyResult object at 0x7fa5981a1cd0>, timeout = None

    def get(self, timeout=None):
        self.wait(timeout)
        if not self.ready():
            raise TimeoutError
        if self._success:
            return self._value
        else:
>           raise self._value
E           TypeError: '>' not supported between instances of 'NoneType' and 'float'
```

