fix traitlets warning->error on 6.5.x (#7051)

* typo in default contents manager class

it's jupyter_server.services.contents, not jupyter_server.contents.services

* fix import of warnings.warn

never part of the traitlets API

Author: minrk

notebook/notebookapp.py

@@ -1408,7 +1408,7 @@ def _update_mathjax_config(self, change):
             # and allow jupyter_server contents managers to pass
             # through. If jupyter_server is not installed, this class
             # will be ignored.
-            'jupyter_server.contents.services.managers.ContentsManager'
+            "jupyter_server.services.contents.managers.ContentsManager",
         ],
         config=True,
         help=_('The notebook manager class to use.')

notebook/traittypes.py

@@ -1,5 +1,6 @@
 import inspect
-from traitlets import ClassBasedTraitType, Undefined, warn
+from warnings import warn
+from traitlets import ClassBasedTraitType, Undefined
 
 # Traitlet's 5.x includes a set of utilities for building
 # description strings for objects. Traitlets 5.x does not