related_name is Optional[str] (#176)

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.

django-stubs/db/models/fields/related.pyi

@@ -114,7 +114,7 @@ class ForeignKey(ForeignObject[_ST, _GT]):
         to: Union[Type[Model], str],
         on_delete: Callable[..., None],
         to_field: Optional[str] = ...,
-        related_name: str = ...,
+        related_name: Optional[str] = ...,
         related_query_name: Optional[str] = ...,
         limit_choices_to: Optional[Union[Dict[str, Any], Callable[[], Any], Q]] = ...,
         parent_link: bool = ...,
@@ -159,7 +159,7 @@ class OneToOneField(RelatedField[_ST, _GT]):
         to: Union[Type[Model], str],
         on_delete: Any,
         to_field: Optional[str] = ...,
-        related_name: str = ...,
+        related_name: Optional[str] = ...,
         related_query_name: Optional[str] = ...,
         limit_choices_to: Optional[Union[Dict[str, Any], Callable[[], Any], Q]] = ...,
         parent_link: bool = ...,