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.

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,
 )

tox.ini

@@ -8,10 +8,10 @@ passenv = SPHINXOPTS
 [testenv:sphinx]
 deps =
   django < 2
-  .[testing]
+  .[building]
   pifpaf
 commands =
-  {envpython} -m pifpaf run postgresql -- make -C docs html
+  {envpython} -m pifpaf run postgresql -- make -C docs {posargs:html}
 
 [testenv:sphinx-dev]
 deps =
@@ -19,7 +19,7 @@ deps =
   -rrequirements-swh-dev.txt
   pifpaf
 commands =
-  {envpython} -m pifpaf run postgresql -- make -C docs html
+  {envpython} -m pifpaf run postgresql -- make -C docs {posargs:html}
 
 [testenv:flake8]
 skip_install = true