fix issues related to jupyter lab

- see jupyter-server/jupyter_server#1348

omegaml/notebook/checkpoints.py

@@ -1,22 +1,42 @@
+from IPython.utils.tz import utcnow
 from jupyter_server.services.contents.checkpoints import GenericCheckpointsMixin, Checkpoints
 
+
 class NoOpCheckpoints(GenericCheckpointsMixin, Checkpoints):
-    """requires the following methods:"""
     # source: https://jupyter-server.readthedocs.io/en/latest/developers/contents.html
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        self.log.info("**** NoOpCheckpoints initialized")
+
+    def checkpoint_model(self):
+        info = {
+            "id": "",
+            "last_modified": utcnow()
+        }
+        return info
+
     def create_file_checkpoint(self, content, format, path):
         """ -> checkpoint model"""
+        return self.checkpoint_model()
+
     def create_notebook_checkpoint(self, nb, path):
         """ -> checkpoint model"""
+        return self.checkpoint_model()
+
     def get_file_checkpoint(self, checkpoint_id, path):
         """ -> {'type': 'file', 'content': <str>, 'format': {'text', 'base64'}}"""
+
     def get_notebook_checkpoint(self, checkpoint_id, path):
         """ -> {'type': 'notebook', 'content': <output of nbformat.read>}"""
+
     def delete_checkpoint(self, checkpoint_id, path):
         """deletes a checkpoint for a file"""
+
     def list_checkpoints(self, path):
         """returns a list of checkpoint models for a given file,
         default just does one per file
         """
         return []
+
     def rename_checkpoint(self, checkpoint_id, old_path, new_path):
         """renames checkpoint from old path to new path"""

omegaml/notebook/omegacontentsmgr.py

@@ -1,9 +1,8 @@
-import mimetypes
-from base64 import encodebytes, decodebytes
-
 import json
+import mimetypes
 import nbformat
 import os
+from base64 import encodebytes, decodebytes
 from datetime import datetime
 from io import BytesIO
 from jupyter_server.services.contents.manager import ContentsManager
@@ -21,17 +20,16 @@ class OmegaStoreContentsManager(ContentsManager):
     Adopted from notebook/services/contents/filemanager.py
 
     This requires a properly configured omegaml instance.
-    see http://jupyter-notebook.readthedocs.io/en/stable/extending/contents.html
+    see https://jupyter-server.readthedocs.io/en/latest/developers/contents.html
     """
 
+    checkpoints_class = NoOpCheckpoints
+
     def __init__(self, **kwargs):
         # pass omega= for testing purpose
         self._omega = kwargs.pop('omega', None)
         super(OmegaStoreContentsManager, self).__init__(**kwargs)
 
-    def _checkpoints_class_default(self):
-        return NoOpCheckpoints
-
     @property
     def omega(self):
         """