Ensure that the doc requirement are set as default for 'Readthedocs'

http://docs.readthedocs.io/en/latest/faq.html#how-do-i-change-behavior-for-read-the-docs

setup.py

@@ -39,6 +39,13 @@
     "pytest-cov >= 2, < 3"
 ]
 
+
+# Readthedocs requires Sphinx extensions to be specified as part of
+# install_requires in order to build properly.
+if os.environ.get("READTHEDOCS", None) == "True":
+    INSTALL_REQUIRES.extend(DOC_REQUIRES)
+
+
 setup(
     name="champollion",
     version=VERSION,