pin to specific Sphinx version and older docutils for building docs

[boegel]

Building the docs fails currently on ReadTheDocs because of an incompatibility with the fixed Sphinx 1.8.5 version and the latest docutils that gets installed (details below).

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/easybuild/envs/develop/lib/python3.7/site-packages/sphinx/cmd/build.py", line 304, in build_main
    app.build(args.force_all, filenames)
  File "/home/docs/checkouts/readthedocs.org/user_builds/easybuild/envs/develop/lib/python3.7/site-packages/sphinx/application.py", line 341, in build
    self.builder.build_update()
  File "/home/docs/checkouts/readthedocs.org/user_builds/easybuild/envs/develop/lib/python3.7/site-packages/sphinx/builders/__init__.py", line 347, in build_update
    len(to_build))
  File "/home/docs/checkouts/readthedocs.org/user_builds/easybuild/envs/develop/lib/python3.7/site-packages/sphinx/builders/__init__.py", line 360, in build
    updated_docnames = set(self.read())
  File "/home/docs/checkouts/readthedocs.org/user_builds/easybuild/envs/develop/lib/python3.7/site-packages/sphinx/builders/__init__.py", line 468, in read
    self._read_serial(docnames)
  File "/home/docs/checkouts/readthedocs.org/user_builds/easybuild/envs/develop/lib/python3.7/site-packages/sphinx/builders/__init__.py", line 490, in _read_serial
    self.read_doc(docname)
  File "/home/docs/checkouts/readthedocs.org/user_builds/easybuild/envs/develop/lib/python3.7/site-packages/sphinx/builders/__init__.py", line 534, in read_doc
    doctree = read_doc(self.app, self.env, self.env.doc2path(docname))
  File "/home/docs/checkouts/readthedocs.org/user_builds/easybuild/envs/develop/lib/python3.7/site-packages/sphinx/io.py", line 318, in read_doc
    pub.publish()
  File "/home/docs/checkouts/readthedocs.org/user_builds/easybuild/envs/develop/lib/python3.7/site-packages/docutils/core.py", line 219, in publish
    self.apply_transforms()
  File "/home/docs/checkouts/readthedocs.org/user_builds/easybuild/envs/develop/lib/python3.7/site-packages/docutils/core.py", line 200, in apply_transforms
    self.document.transformer.apply_transforms()
  File "/home/docs/checkouts/readthedocs.org/user_builds/easybuild/envs/develop/lib/python3.7/site-packages/sphinx/transforms/__init__.py", line 90, in apply_transforms
    Transformer.apply_transforms(self)
  File "/home/docs/checkouts/readthedocs.org/user_builds/easybuild/envs/develop/lib/python3.7/site-packages/docutils/transforms/__init__.py", line 171, in apply_transforms
    transform.apply(**kwargs)
  File "/home/docs/checkouts/readthedocs.org/user_builds/easybuild/envs/develop/lib/python3.7/site-packages/sphinx/transforms/__init__.py", line 245, in apply
    apply_source_workaround(n)
  File "/home/docs/checkouts/readthedocs.org/user_builds/easybuild/envs/develop/lib/python3.7/site-packages/sphinx/util/nodes.py", line 94, in apply_source_workaround
    for classifier in reversed(node.parent.traverse(nodes.classifier)):
TypeError: 'generator' object is not reversible

The problem is discussed in sphinx-doc/sphinx#9727, pinning to an old docutils version should fix the problem. Sphinx is also being pinned to 1.8.5, since that's the default for old ReadTheDocs projects (see https://docs.readthedocs.io/en/stable/builds.html#python).