documentation: nightly buid fails with "RuntimeError: Working outside of request context"

As per title, (not so) longer traceback is:

Initiated sphinxcontrib-images backend: `sphinxcontrib_images_lightbox2.lightbox2:LightBox2`
building [mo]: targets for 0 po files that are out of date
building [html]: targets for 522 source files that are out of date
updating environment: [new config] 522 added, 0 changed, 0 removed
reading sources... [  0%] apidoc/modules
reading sources... [  0%] apidoc/swh
reading sources... [  0%] apidoc/swh.core
reading sources... [  0%] apidoc/swh.core.api

Exception occurred:
  File "/home/jenkins/workspace/DDOC/publish/swh-docs/.tox/sphinx/lib/python3.7/site-packages/flask/globals.py", line 38, in _lookup_req_object
    raise RuntimeError(_request_ctx_err_msg)
RuntimeError: Working outside of request context.

This typically means that you attempted to use functionality that needed
an active HTTP request.  Consult the documentation on testing for
information about how to avoid this problem.
The full traceback has been saved in /tmp/sphinx-err-pof5su3o.log, if you want to report the issue to the developers.

Full build log: https://jenkins.softwareheritage.org/job/DDOC/job/publish/543/console

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Migrated from T2380 (view on Phabricator)