Stop using deprecated function in tornado

The `add_callback_from_signal` is deprecated in tornado [6.4](https://github.com/tornadoweb/tornado/blob/a6dfd70d7a6398b2187021df63ca77daa5781e5e/docs/releases/v6.4.0.rst#deprecation-notices) and causing our builds to fail from the warning.

Failing build example: https://github.com/jupyter-server/jupyter_server/actions/runs/7092240415/job/19303106530?pr=1370
