Update the intersphinx_utils extension for Sphinx 3+
Review Request #11485 — Created Feb. 25, 2021 and submitted — Latest diff uploaded
In Sphinx 3 and up, the listeners for missing references are represented
as a list. Earlier versions use a dictionary. This change updates the
code to check for either form, bringing compatibility to both.
Been using this to build docs on Sphinx 2 and 3.