Add a workaround to fix Read the Docs builds

The latest version of docutils (0.18) causes Sphinx to raise an error.
See https://github.com/sphinx-doc/sphinx/issues/9727 for details.

requirements-rtd.txt

-Pygments==2.6.1
+# NOTE: pin docutils to fix ReadTheDocs builds.
+# See https://github.com/sphinx-doc/sphinx/issues/9727
+docutils<0.18