Building the docs breaks (unexpected unindent)

[plamut]

I noticed that building the docs locally on master fails due to the following error:

sphinx.errors.SphinxWarning: <workspace_dir>/python-pubsub/docs/changelog.md:416:Bullet list ends without a blank line; unexpected unindent.

However, it turns out that the issue does not occur with the older version of Sphinx (< 3.x), thus it appears as a bug in the Sphinx package itself. A workaround is to pin that dependency to an older version, but preferably all new warnings emitted in Sphinx 3.x should be addressed at their sources.