low-frequency error on module object with `use_dill`

Apparently there's a low-frequency issue (submitted by @jakirkham) that surfaces with code similar to this:

```
from ipyparallel import Client

client = Client()
client[:].use_dill().get()

with client[:].sync_imports():
import nest
from nest import A, B
```

Which produces:

```
AttributeError: 'module' object has no attribute '__main__'
```

I have not been able to reproduce the error, however @tcwalther has -- and provides a workaround.

See https://github.com/uqfoundation/dill/issues/133.

To me, it smells like something `ipyparallel` should be handling… so, I'm punting this over to an `ipyparallel` ticket until I hear differently.

Start by running the issue's Client, use_dill(), and sync_imports() example, then compare the failure with dill issue 133. No source file or test is named; done means determining whether ipyparallel owns the error and documenting or fixing the behavior accordingly.

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