Do not require post_save to return None (#74)

It can return any value, since it is ignored

django_fakery/types.py

@@ -7,6 +7,6 @@
 FieldMap = Dict[str, Any]
 Lookup = Dict[str, Any]
 SaveHooks = List[Callable[[T], None]]
-LazySaveHooks = List[Callable[[models.Model], None]]
+LazySaveHooks = List[Callable[[models.Model], Any]]
 Built = Tuple[T, Dict[str, List[models.Model]]]
 LazyBuilt = Tuple[models.Model, Dict[str, List[models.Model]]]