Setup doctests to check Python snippets in docs

Ideally #473 should have been caught by an automated tests (using doctest or similar)

http://www.sphinx-doc.org/en/stable/ext/doctest.html

Start by reviewing issue #473 and the linked Sphinx doctest documentation to understand the reported Python snippet failure and the proposed approach. Find the documentation build entry point and determine how doctests should be invoked; done means the documentation snippets are automatically checked and the reported problem is caught.

Written by the indexing model from the issue text.