Pin docutils<0.18, Sphinx doesn't work with newer version

This should fix the readthedocs.io failing builds.

Author: Photonios

setup.py

@@ -68,7 +68,7 @@ def run(self):
     ],
     extras_require={
         ':python_version <= "3.6"': ["dataclasses"],
-        "docs": ["Sphinx==2.2.0", "sphinx-rtd-theme==0.4.3"],
+        "docs": ["Sphinx==2.2.0", "sphinx-rtd-theme==0.4.3", "docutils<0.18"],
         "test": [
             "psycopg2==2.8.4",
             "dj-database-url==0.5.0",