Fixed html sidebars to supported version in Sphinx

[AdaBehan]

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

Closes #1565

Pull Request Checklist

• Changes have tests
• News fragment added in changelog.d. See documentation for details

[pganssle]

I have rebased this on #1802, since this is currently untested in the CI.

[pganssle]

Are we sure this is working correctly? The sidebar on the netlify preview looks different from the sidebar on the current documentation.

The current documentation has "Next topic" and "This Page", both of which are missing in the netlify version.

[AdaBehan]

@pganssle well spotted it does not look right. I will have a look.

[AdaBehan]

@pganssle Little unsure what todo about the list method not being supported in older versions of Sphinx.

4:59:00 PM: Running Sphinx v1.8.5
4:59:00 PM: Configuration error:
4:59:00 PM: There is a syntax error in your configuration file: invalid syntax (conf.py, line 72)
4:59:01 PM: ERROR: InvocationError for command /opt/build/repo/.tox/docs/bin/sphinx-build -W -b html -d /opt/build/repo/.tox/docs/tmp/doctrees docs docs/build/html (exited with code 2)
4:59:01 PM: ___________________________________ summary ____________________________________
4:59:01 PM: ERROR:   docs: commands failed

I could do something like:

if Sphinx.__version__  > 2 : 
    html_sidebars = {'index': ['indexsidebar.html']}
else:
    html_sidebars = {'index': ['relations.html', 'sourcelink.html', 'indexsidebar.html', 'searchbox.html']}

Maybe i am miss reading the Sphenix documentation but it does not look like there is another way to declare the "value".
https://www.sphinx-doc.org/en/master/usage/configuration.html#confval-html_sidebars

[pganssle]

@adamRogerson That doesn't seem right, Sphinx was raising deprecation warnings in version 1.8.5, so they must have had a migration target in mind.

Are you sure the list version is not supported? The PR seems to be succeeding now and the netlify build (which uses 1.8.5) is doing the right thing.

[AdaBehan]

@pganssle I must have misread the error there. Is there anything else that needs to be done on this ?