gh-36506: Support launching notebook 7
As reported in #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: #36414
URL: https://github.com/sagemath/sage/pull/36506
Reported by: Gonzalo Tornaría
Reviewer(s): Matthias Köppe
