Fix SyntaxWarning from update message

The use of backslashes in the ASCII-art "Update" banner results in an "invalid escape sequence" SyntaxWarning under Python 3.12.  Fix by changing the banner string to a raw string (which ignores backslashes).

notebook/notebookapp.py

@@ -185,7 +185,7 @@ def __init__(self, jupyter_app, kernel_manager, contents_manager,
         if settings['autoreload']:
             log.info('Autoreload enabled: the webapp will restart when any Python src file changes.')
 
-        print("""
+        print(r"""
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