Put swh dependencies in a dedicated extra requirements
to prevent weird race condition when both requirements-swh.txt and requirements-swh-dev.txt are added as requirements.
This is needed to make:
tox -e sphinx-dev
work as expected, ie. with swh packages installed from sources rather than from pypi.
Test Plan
both tox envs 'sphinx' and 'sphinx-dev' should work as expected
especially, sphinx-dev should not generate a traceback when autodoc tries to import swh.deposit.settings.production if ../swh-deposit is on current master.
Migrated from D735 (view on Phabricator)