sagemathgh-36506: Support launching notebook 7

    
As reported in sagemath#36414, running `sage -notebook` fails with jupyter
notebook 7.

This seems to be just a matter of a changed import, which we fix.

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Fixes: sagemath#36414
    
URL: sagemath#36506
Reported by: Gonzalo Tornaría
Reviewer(s): Matthias Köppe

src/bin/sage-notebook

@@ -29,7 +29,12 @@ class NotebookJupyter():
     def __init__(self, argv):
         self.print_banner()
         try:
-            from notebook.notebookapp import main
+            try:
+                # notebook 6
+                from notebook.notebookapp import main
+            except ImportError:
+                # notebook 7
+                from notebook.app import main
         except ImportError:
             import traceback
             traceback.print_exc()