swh.web.settings.production: Fix mypy warning

Fix ci [1]

Also the setup is correctly configured as per drf's doc [2]

• [1] https://jenkins.softwareheritage.org/job/DWAPPS/job/tests/1182/console

• [2] https://www.django-rest-framework.org/api-guide/settings/#num_proxies

Depends on !1000 (closed)

Test Plan

tox -e mypy

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Migrated from D2833 (view on Phabricator)