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 https://github.com/sphinx-doc/sphinx/issues/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).