"maximum recursion depth exceeded " when serializing enum

After `pip2.7 install enum34 dill` running the following script with python2.7

```
import dill
import enum

class SomeEnumType(enum.Enum):
  VALUE_A = "value_a"
  VALUE_B = "value_b"

dill.dumps(SomeEnumType.VALUE_A)
```

fails with
RuntimeError: maximum recursion depth exceeded while calling a Python object


the same script under python3 works fine.
