rtd: Fix warnings in RTD causing autodoc to fail

This patch set fixes warnings [0] in RTD which are causing autodoc
to fail to build, resulting in missing documentation as in
here: https://airship-deckhand.readthedocs.io/en/latest/operators/exceptions.html

Example warnings:

WARNING: autodoc: failed to import module 'deckhand.policy'; the following exception was raised:
No module named 'falcon'

[0] https://readthedocs.org/api/v2/build/8001684.txt

Change-Id: I872b90e03a8f9e3de3fd717ed340b53f095f01a5

doc/requirements.txt

@@ -2,11 +2,7 @@
 # of appearance. Changing the order has an impact on the overall integration
 # process, which may cause wedges in the gate later.
 sphinx>=1.6.2 # BSD
-sphinx_rtd_theme==0.2.4
+sphinx_rtd_theme
 reno>=2.5.0 # Apache-2.0
 plantuml
 
-# NOTE(fmontei): The requirement below is only included because readthedocs
-# depends on it to work, since it only takes in one requirements file.
-oslo.config!=4.3.0,!=4.4.0,>=5.2.0 # Apache-2.0
-oslo.policy>=1.33.1 # Apache-2.0

doc/source/conf.py

@@ -18,8 +18,13 @@
 #
 import os
 import sys
-sys.path.insert(0, os.path.abspath('.'))
+
+# If extensions (or modules to document with autodoc) are in another directory,
+# add these directories to sys.path here. If the directory is relative to the
+# documentation root, use os.path.abspath to make it absolute, like shown here.
+sys.path.insert(0, os.path.abspath('../../'))
 sys.path.insert(0, os.path.abspath('../'))
+sys.path.insert(0, os.path.abspath('./'))
 
 # -- General configuration ------------------------------------------------