build: fix warnings in docgen (#711)

Resolved all the outstanding warnings from sphinx.

Going forward, warnings will be treated as errors since the `-W` flag is being used with `sphinx-build` 
diff --git a/docs/conf.py b/docs/conf.py
index db1872e..b68467f 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -369,7 +369,7 @@
     "python": ("https://docs.python.org/3.5", None),
     "urllib3": ("https://urllib3.readthedocs.io/en/stable", None),
     "requests": ("https://requests.kennethreitz.org/en/master/", None),
-    "requests-oauthlib": ("http://requests-oauthlib.readthedocs.io/en/stable", None),
+    "requests-oauthlib": ("https://requests-oauthlib.readthedocs.io/en/stable/", None),
 }
 
 # Autodoc config