doc generation broken with sphinx < 1.5

Developer documentation no longer builds (this time at all, rather than just throwing ignorable errors), apparently due to swh-deposit. Here's the traceback:

~/swh-environment/swh-docs/docs $ make
Running Sphinx v1.6.4
loading translations [en]... done
Generating grammar tables from /usr/share/sphinx/pycode/Grammar-py3.txt
loading pickled environment... done
loading intersphinx inventory from https://docs.python.org/3/objects.inv...
building [mo]: targets for 0 po files that are out of date
building [html]: targets for 11 source files that are out of date
updating environment: [extensions changed] 133 added, 0 changed, 1 removed
DEPOSIT_STATUS True[  9%] swh-deposit/apidoc/modules                                           
content True
directory True
executable_content True
revision True
symlink True
ALGORITHMS True
DEFAULT_ALGORITHMS True
HASH_BLOCK_SIZE True
type True

Exception occurred:
  File "/usr/lib/python3/dist-packages/sphinxcontrib/httpdomain.py", line 439, in http_header_role
    msg = inliner.reporter.error('%s is not unknown HTTP header' % header,
AttributeError: 'NoneType' object has no attribute 'reporter'
The full traceback has been saved in /tmp/sphinx-err-myex0h6h.log, if you want to report the issue to the developers.
Please also report this if it was a user error, so that a better error message can be provided next time.
A bug report can be filed in the tracker at <https://github.com/sphinx-doc/sphinx/issues>. Thanks!
Makefile:39: recipe for target 'sphinx/html' failed
make: *** [sphinx/html] Error 1

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Migrated from T810 (view on Phabricator)