`check_release (link_check)` CI job is failing

[RRosio]

Description

The CI job Check Release / check_release (link_check) (pull_request) is now more consistently failing

Context

FF                                                                       [ 50%]
=================================== FAILURES ===================================
_ /home/runner/work/nbclassic/nbclassic/tools/tests/Confined Output.ipynb: http://place-hold.it/800x200.png _
http://place-hold.it/800x200.png: HTTPConnectionPool(host='place-hold.it', port=80): Max retries exceeded with url: /800x200.png (Caused by ConnectTimeoutError(<urllib3.connection.HTTPConnection object at 0x7f20decc5340>, 'Connection to place-hold.it timed out. (connect timeout=None)'))
_ /home/runner/work/nbclassic/nbclassic/tools/tests/Confined Output.ipynb: http://place-hold.it/800x200.png _
http://place-hold.it/800x200.png: HTTPConnectionPool(host='place-hold.it', port=80): Max retries exceeded with url: /800x200.png (Caused by ConnectTimeoutError(<urllib3.connection.HTTPConnection object at 0x7f20decc6810>, 'Connection to place-hold.it timed out. (connect timeout=None)'))
=============================== warnings summary ===============================
../../../../../opt/hostedtoolcache/Python/3.12.11/x64/lib/python3.12/site-packages/jupyter_client/connect.py:22
  /opt/hostedtoolcache/Python/3.12.11/x64/lib/python3.12/site-packages/jupyter_client/connect.py:22: DeprecationWarning: Jupyter is migrating its paths to use standard platformdirs
  given by the platformdirs library.  To remove this warning and
  see the appropriate new directories, set the environment variable
  `JUPYTER_PLATFORM_DIRS=1` and then run `jupyter --paths`.
  The use of platformdirs will be the default in `jupyter_core` v6
    from jupyter_core.paths import jupyter_data_dir, jupyter_runtime_dir, secure_write

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
=========================== short test summary info ============================
FAILED tools/tests/Confined Output.ipynb::Cell 1 <img src=http://place-hold.it/800x200.png>
FAILED tools/tests/Confined Output.ipynb::Cell 1 <img src=http://place-hold.it/800x200.png>
2 failed, 1 warning in 269.60s (0:04:29)