When trying to serialize some of the dynamically generated modules that pytest generates, errors like these are encountered.

```
  File "/Users/mvanniekerk/miniconda3/envs/py36/lib/python3.6/pickle.py", line 476, in save
    f(self, obj) # Call unbound method with explicit self
  File "/Users/mvanniekerk/src/pytest-dask/.tox/py36/lib/python3.6/site-packages/cloudpickle/cloudpickle.py", line 366, in save_function
    self.save_function_tuple(obj)
  File "/Users/mvanniekerk/src/pytest-dask/.tox/py36/lib/python3.6/site-packages/cloudpickle/cloudpickle.py", line 494, in save_function_tuple
    itertools.chain(f_globals.values(), closure_values or ()),
  File "/Users/mvanniekerk/src/pytest-dask/.tox/py36/lib/python3.6/site-packages/cloudpickle/cloudpickle.py", line 388, in _save_subimports
    if isinstance(x, types.ModuleType) and x.__package__:
  File "/Users/mvanniekerk/src/pytest-dask/.tox/py36/lib/python3.6/site-packages/py/_apipkg.py", line 123, in __makeattr
    raise AttributeError(name)
AttributeError: __package__
```

After applying this change the resulting serialized objects work correctly