"tox -e docs" fail due to docutils 0.13.1

1.Action
when executing tox -e docs, throw exception

2.Stack
Traceback (most recent call last):
  File "setup.py", line 29, in <module>
    pbr=True)
  File "/usr/lib64/python2.7/distutils/core.py", line 152, in setup
    dist.run_commands()
  File "/usr/lib64/python2.7/distutils/dist.py", line 953, in run_commands
    self.run_command(cmd)
  File "/usr/lib64/python2.7/distutils/dist.py", line 972, in run_command
    cmd_obj.run()
  File "/home/openstack-git/taskflow/.tox/docs/lib/python2.7/site-packages/pbr/builddoc.py", line 196, in run
    self._sphinx_run()
  File "/home/openstack-git/taskflow/.tox/docs/lib/python2.7/site-packages/pbr/builddoc.py", line 150, in _sphinx_run
    app.build(force_all=self.all_files)
  File "/home/openstack-git/taskflow/.tox/docs/lib/python2.7/site-packages/sphinx/application.py", line 261, in build
    self.builder.build_all()
  File "/home/openstack-git/taskflow/.tox/docs/lib/python2.7/site-packages/sphinx/builders/__init__.py", line 211, in build_all
    self.build(None, summary='all source files', method='all')
  File "/home/openstack-git/taskflow/.tox/docs/lib/python2.7/site-packages/sphinx/builders/__init__.py", line 322, in build
    self.write(docnames, list(updated_docnames), method)
  File "/home/openstack-git/taskflow/.tox/docs/lib/python2.7/site-packages/sphinx/builders/__init__.py", line 360, in write
    self._write_serial(sorted(docnames), warnings)
  File "/home/openstack-git/taskflow/.tox/docs/lib/python2.7/site-packages/sphinx/builders/__init__.py", line 368, in _write_serial
    self.write_doc(docname, doctree)
  File "/home/openstack-git/taskflow/.tox/docs/lib/python2.7/site-packages/sphinx/builders/html.py", line 443, in write_doc
    self.docwriter.write(doctree, destination)
  File "/home/openstack-git/taskflow/.tox/docs/lib/python2.7/site-packages/docutils/writers/__init__.py", line 80, in write
    self.translate()
  File "/home/openstack-git/taskflow/.tox/docs/lib/python2.7/site-packages/sphinx/writers/html.py", line 53, in translate
    self.document.walkabout(visitor)
  File "/home/openstack-git/taskflow/.tox/docs/lib/python2.7/site-packages/docutils/nodes.py", line 187, in walkabout
    visitor.dispatch_departure(self)
  File "/home/openstack-git/taskflow/.tox/docs/lib/python2.7/site-packages/docutils/nodes.py", line 1895, in dispatch_departure
    return method(node)
  File "/home/openstack-git/taskflow/.tox/docs/lib/python2.7/site-packages/docutils/writers/_html_base.py", line 671, in depart_document
    assert not self.context, 'len(context) = %s' % len(self.context)
AssertionError: len(context) = 1

3.Related information
the link is about talking about this problem. it seems that we should waiting for sphinx to update
https://github.com/sphinx-doc/sphinx/issues/3212