Add swagger_fake_view marker to help detect fake views in get_serializer

Cleaner fix for #154

docs/openapi.rst

@@ -174,6 +174,15 @@ in a few ways:
   :ref:`@swagger_auto_schema <custom-spec-swagger-auto-schema>`, to prevent ``get_serializer`` from being called on
   the view
 * :ref:`exclude your endpoint from introspection <custom-spec-excluding-endpoints>`
+* use the ``swagger_fake_view`` marker to detect requests generated by ``drf-yasg``:
+
+   .. code-block:: python
+
+     def get_serializer_class(self):
+        if getattr(self, 'swagger_fake_view', False):
+           return TodoTreeSerializer
+
+        raise NotImplementedError("must not call this")
 
 .. _SCRIPT_NAME: https://www.python.org/dev/peps/pep-0333/#environ-variables
 .. _FORCE_SCRIPT_NAME: https://docs.djangoproject.com/en/2.0/ref/settings/#force-script-name