Update notebook in assignment check for python 3.8

Author: shreve

nbgrader/converters/generate_assignment.py

@@ -101,7 +101,7 @@ def _clean_old_notebooks(self, assignment_id: str, student_id: str) -> None:
             for notebook in self.notebooks:
                 notebook = Path(notebook)
 
-                if not notebook.is_relative_to(assignment_dir):
+                if assignment_dir not in notebook.parents:
                     continue
 
                 new_notebook_ids.add(notebook.stem)