[V3 Docs] Third time's the charm (#1447)

* Get the right sphinx version

* Add mongo extra

.readthedocs.yml

@@ -10,4 +10,5 @@ python:
   version: 3.6
   pip_install: true
   extra_requirements:
-    - docs
+    - docs
+    - mongo

docs/requirements.txt

@@ -1,4 +1,4 @@
-sphinx==1.6.3
+sphinx==1.6.5
 sphinxcontrib-asyncio
 sphinx_rtd_theme
 git+https://github.com/Rapptz/discord.py@rewrite#egg=discord.py[voice]

setup.py

@@ -130,7 +130,7 @@ setup(
             'pytest>3', 'pytest-asyncio'
         ],
         'mongo': ['motor'],
-        'docs': ['sphinx', 'sphinxcontrib-asyncio', 'sphinx_rtd_theme'],
+        'docs': ['sphinx==1.6.5', 'sphinxcontrib-asyncio', 'sphinx_rtd_theme'],
         'voice': ['red-lavalink>=0.0.4']
     }
 )