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.

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.