Allow bool or string for relationship(passive_deletes=...) arg (#143)

The relationship(passive_deletes=...) can be either a boolean or a string:

https://docs.sqlalchemy.org/en/13/orm/relationship_api.html#sqlalchemy.orm.relationship.params.passive_deletes

Author: wbolster

sqlalchemy-stubs/orm/relationships.pyi

@@ -31,7 +31,7 @@ class RelationshipProperty(StrategizedProperty, Generic[_T_co]):
     lazy: Any = ...
     single_parent: bool = ...
     collection_class: Any = ...
-    passive_deletes: bool = ...
+    passive_deletes: Union[bool, str] = ...
     cascade_backrefs: bool = ...
     passive_updates: bool = ...
     remote_side: Any = ...
@@ -61,7 +61,7 @@ class RelationshipProperty(StrategizedProperty, Generic[_T_co]):
                  back_populates: Optional[Any] = ..., post_update: bool = ..., cascade: Union[str, bool] = ...,
                  extension: Optional[Any] = ..., viewonly: bool = ...,
                  lazy: Optional[Union[str, bool]] = ..., collection_class: Optional[Any] = ...,
-                 passive_deletes: bool = ..., passive_updates: bool = ...,
+                 passive_deletes: Union[bool, str] = ..., passive_updates: bool = ...,
                  remote_side: Optional[Any] = ..., enable_typechecks: bool = ...,
                  join_depth: Optional[Any] = ..., comparator_factory: Optional[Any] = ...,
                  single_parent: bool = ..., innerjoin: bool = ..., distinct_target_key: Optional[Any] = ...,