tox: add a sphinx-dev tox environment

that builds the doc using source code repos located one directory above
instead of published versions of swh subpackages.

Related to T1330.

requirements-swh-dev.txt

+# Add here internal Software Heritage dependencies, one per line.
+../swh-core
+../swh-model
+../swh-objstorage[testing]
+../swh-scheduler
+../swh-storage[schemadata]
+../swh-loader-core
+../swh-lister
+../swh-journal
+../swh-vault
+../swh-loader-dir
+../swh-loader-tar
+../swh-loader-pypi
+../swh-loader-debian
+../swh-loader-mercurial
+../swh-loader-svn
+../swh-loader-git
+../swh-archiver
+../swh-web
+../swh-deposit
+../swh-indexer
+../swh-mirror-forge

tox.ini

@@ -9,6 +9,14 @@ deps =
 commands =
   {envpython} -m pifpaf run postgresql -- make -C docs html
 
+[testenv:sphinx-dev]
+deps =
+  django < 2
+  -r requirements-swh-dev.txt
+  pifpaf
+commands =
+  {envpython} -m pifpaf run postgresql -- make -C docs html
+
 [testenv:flake8]
 skip_install = true
 deps =