DjangoFormMutation does not generate enum from forms.ChoiceField

* **What is the current behavior?**
``` python

GEEKS_CHOICES =(("1", "One"), ("2", "Two"))

class UpdateUserStateInTradeForm(forms.Form):
state = forms.ChoiceField(choices=GEEKS_CHOICES)

# mutation
class UpdateUserStateInTradeMutation(DjangoFormMutation):
class Meta:
form_class = UpdateUserStateInTradeForm

# generated schema

state: String!
```

* **What is the expected behavior?**

state is enumeration

* **Please tell us about your environment:**

- Version: 2.13.0, Django 3.0.1
- Platform: Linux