Update Sphinx to work with docutils-0.18.x

[ViktorHaag]

Is your feature request related to a problem? Please describe.
Docutils 0.18.0 has been released and at first glance it appears to have changes in it that are incompatible with Sphinx (cloning the Sphinx repo, running the tests clean; then installing docutils 0.18, updating Sphinx to allow docutils-0.18 and include it in the test suite, shows a whole bunch of failed tests).

Describe the solution you'd like
Make the changes to Sphinx required to be compatible with docutils 0.18.x.

Describe alternatives you've considered
The alternative would be to remain pinned to 0.17.x (ie "docutils packages with a version less than 0.18"); this may cause disruptions in projects and platforms that assume they can freely update dependencies to the latest versions, especially if they directly depend on docutils.

[astrojuanlu]

Some changes have been made already to support docutils 0.18, see for example #9701, #9724, #9725. However, it's still pinned to <0.18:

sphinx/setup.py

Line 26 in 2be9d6b

'docutils>=0.14,<0.18',

[nicobrevin]

Just in case anyone else is on an old version of sphinx, I had to pin my docutils version like so:

 sphinx==1.7.6
+docutils==0.17.1
 doc8==0.5.0
 sphinx_rtd_theme==0.4.1
 m2r==0.1.15

Without it I was getting this error:

Exception occurred:
  File "/usr/local/lib/python2.7/dist-packages/sphinx/util/nodes.py", line 57, in apply_source_workaround
    for classifier in reversed(node.parent.traverse(nodes.classifier)):
TypeError: argument to reversed() must be a sequence

The version constraint is pretty loose in that version of sphinx:

Collecting docutils>=0.11 (from sphinx==1.7.6->-r /tmp/requirements.txt (line 1))

Hopefully this will help another weary traveller searching for a solution after their build suddenly failed...