IntegerChoices does not output correct Enum

When generating a schema from a model that has as a field of type `IntegerChoices` https://docs.djangoproject.com/en/3.0/ref/models/fields/#choices), the choices are not displayed in the schema

* **What is the current behavior?**
```python
from django.db import models
from graphene_django.types import DjangoObjectType

# Model
class ExampleModel(models.Model):
class Answer(models.IntegerChoices):
NO = 0
YES = 1

#Schema
class ExampleType(DjangoObjectType):
class Meta:
model = ExampleModel
```

When generating the schema with `./manage.py graphql_schema --schema example_app.schema.root_schema --out schema.graphql` the output looks like this

```graphql
enum ExampleType {
A_0
A_1
}
```

`YES` and `NO` are not displayed in the enum

* **What is the expected behavior?**

`YES` and `NO` are displayed in the enum
```graphql
enum ExampleType {
YES
NO
}
```

* **What is the motivation / use case for changing the behavior?**

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

- Version:
Python 3.7
graphene-django==2.10.1
graphene==2.1.8
Django==3.0.6
- Platform:
MacOs

* **Other information** (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. stackoverflow)