deposit: apidoc generation fails on settings.production module with traceback

when generating documentation for swh-deposit, the following unhandled traceback makes sphinx skip the module in title:

/home/zack/dati/projects/sw-heritage/git/swh-environment/swh-deposit/docs/apidoc/swh.deposit.settings.rst:28: WARNING: autodoc: failed to import module 'swh.deposit.settings.production'; the following exception was raised:
Traceback (most recent call last):
  File "/home/zack/.local/lib/python3.6/site-packages/sphinx/ext/autodoc.py", line 658, in import_object
    __import__(self.modname)
  File "/home/zack/dati/projects/sw-heritage/git/swh-environment/swh-deposit/swh/deposit/settings/production.py", line 29, in <module>
    SECRET_KEY = private_conf['secret_key']
KeyError: 'secret_key'

Migrated from T1182 (view on Phabricator)

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