In the current Django (2.1, 2.2), the `related_name` parameter to
`OneToOneField` and `ForeignKey` classes (much like `ManyToManyField`
and the base `ForeignObject` classes) is `Optional[str]`. While
it's true that most authors won't pass `None` here, derived field
types will want to be able to do so.
This ensures that the order in which these mixins are included
into a derrived class does not matter and ends up more accurately
reflecting the return type of SuccessMessageMixin in the process
(its code doesn't appear to enforce that the returned response
is a redirect).
This provides a fix to a secondary aspect of
https://github.com/mkurnikov/django-stubs/issues/79.
