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)

setup.py

@@ -29,9 +29,10 @@ setup(
     url='https://forge.softwareheritage.org/source/swh-docs/',
     packages=find_packages(),
     scripts=[],
-    install_requires=parse_requirements() + parse_requirements('swh'),
+    install_requires=parse_requirements(),
     setup_requires=['vcversioner'],
-    extras_require={'testing': parse_requirements('test')},
+    extras_require={'testing': parse_requirements('test'),
+                    'building': parse_requirements('swh')},
     vcversioner={},
     include_package_data=True,
 )