I use middleware for that right now

I use middleware for that right now

```python
class HideIntrospectMiddleware:
"""
This middleware should use for production mode. This class hide the
introspection.
"""
def resolve(self, next, root, info, **args):
if info.field_name == '__schema':
return None
return next(root, info, **args)
```

Originally posted by @Arfey in a [Graphene issue](https://github.com/graphql-python/graphene/issues/907#issuecomment-484014679), but it gives an error "Cannot return null for non-nullable field Query.__schema."