Pin Sphinx 3.x to docutils <0.17

This will address the issues that the latest docutils release caused.
I think thi si a good practice in general to make sure we have a defined range of docutils versions,
given that they might change in the future.
Having this defined will mean that 3.x versions of Sphinx will always work,
even when docutils has advanced with additional backwards incompatible features.

Refs #9065 #9063 #9061 #9051

setup.py

@@ -23,7 +23,7 @@ install_requires = [
     'sphinxcontrib-qthelp',
     'Jinja2>=2.3',
     'Pygments>=2.0',
-    'docutils>=0.12',
+    'docutils>=0.12,<0.17',
     'snowballstemmer>=1.1',
     'babel>=1.3',
     'alabaster>=0.7,<0.8',