EnumField's should inherit from common base titled EnumField

Right now to determine if a field is an EnumField the best option is to do something like:

```python

from django_enum.fields import EnumMixin

isinstance(Model._meta.get_field('field'), EnumMixin)
```

This is not documented and not intuitive given that EnumFields are defined as EnumField. Should be able to make this test with EnumField rather than EnumMixin.