fix(obtain_auth_token): Fix openapi schema generation

[gnuletik]

This PR fixes the AutoSchema generation for the obtain_auth_token view.

Here is the invalid (partial) OpenAPI schema currently generated.

  /api-token-auth/:
    post:
      operationId: CreateObtainAuthToken
      description: ''
      parameters: []
      requestBody:
        content:
          application/x-www-form-urlencoded:
            schema: &id029 {}
          multipart/form-data:
            schema: *id029
          application/json:
            schema: *id029
      responses:
        '200':
          content:
            application/json:
              schema: {}
          description: ''

As you can see the requestBody's schema and response's schema are empty.
This is because:

• The ObtainAuthToken class inherit from rest_framework.views.APIView which do not have a get_serializer method (this is what is used by the rest_framework.schemas.openapi.AutoSchema class to get the serializer).
• The AuthTokenSerializer did not had the token field.

With this PR, the generated schema is

  /api-token-auth/:
    post:
      operationId: CreateObtainAuthToken
      description: ''
      parameters: []
      requestBody:
        content:
          application/x-www-form-urlencoded:
            schema: &id029
              type: object
              properties:
                username:
                  type: string
                  writeOnly: true
                password:
                  type: string
                  writeOnly: true
                required:
                  - username
                  - password
          multipart/form-data:
            schema: *id029
          application/json:
            schema: *id029
      responses:
        '200':
          content:
            application/json:
              schema:
                type: object
                properties:
                  token:
                    type: string
                    readOnly: true
                required: []
          description: ''

NB: In the ObtainAuthToken class, the schema field (ManualSchema , for coreapi only) was override when the coreapi dependency was installed.
Instead of checking the installed dependency, I replaced the check with coreapi_schema.is_enabled.
This change enables us to run the test script (where coreapi is installed) with the AutoSchema, unless coreapi is explicitly configured.

[carltongibson]

OK, fair, enough. Just one comment:

[carltongibson]

Instead of this, can you just define get_serializer(), and then use that in post()?

[gnuletik]

Definitely! This is done :)

[carltongibson]

Thanks again @gnuletik! Great effort 🔥

[gnuletik]

It's a pleasure :)
Thanks for your time!

[ayetorello]

This fix breaks my login view implementation:

class LoginView(viewsets.ViewSet):
    serializer_class = AuthTokenSerializer
    
    def create(self, request):
        return ObtainAuthToken().post(request)

AttributeError: 'ObtainAuthToken' object has no attribute 'request'