Bug fixes from Zenodo

Note on requirements: 
* python-dateutil  now supports both Python 2 and 3.
* WTForms 2.0: Flask-Admin + Flask-WTF both supports 2.0 now.
* Unpined Redis: Tested on Zenodo.
```python
class InvalidDepositionAction(DepositionError):

    """Raise when ... action."""

```