Merge pull request #1647 from brichet/remove_notebook_dependency

Remove a remaining dependency to notebook

nbgrader/server_extensions/assignment_list/handlers.py

@@ -8,7 +8,7 @@
 from tornado import web
 from textwrap import dedent
 
-from notebook.utils import url_path_join as ujoin
+from jupyter_server.utils import url_path_join as ujoin
 from jupyter_server.base.handlers import JupyterHandler
 from traitlets import Unicode, default
 from traitlets.config import LoggingConfigurable, Config