Fixed html sidebars to supported version in Sphinx (#1804)

.travis.yml

@@ -21,6 +21,8 @@ jobs:
   - <<: *latest_py3
     env: LANG=C
   - python: 3.8-dev
+  - <<: *latest_py3
+    env: TOXENV=docs DISABLE_COVERAGE=1
   - <<: *default_py
     stage: deploy (to PyPI for tagged commits)
     if: tag IS present

changelog.d/1565.misc.rst

+Changed html_sidebars from string to list of string as per
+https://www.sphinx-doc.org/en/master/changes.html#id58 

docs/conf.py

@@ -69,7 +69,7 @@ html_theme_path = ['_theme']
 html_use_smartypants = True
 
 # Custom sidebar templates, maps document names to template names.
-html_sidebars = {'index': 'indexsidebar.html'}
+html_sidebars = {'index': ['relations.html', 'sourcelink.html', 'indexsidebar.html', 'searchbox.html']}
 
 # If false, no module index is generated.
 html_use_modindex = False