Python 3 is used in executing 'tox -e docs' by default currently.
When Python 3 is used, there are some missing notification examples.
In python 3, map function returns an iterator instead of a list,
and importlib.import_module is not executed in the document generation.
So it causes missing versioned notification examples in the nova docs.
This patch fixes it.
Change-Id: Ie4f3f9be0ca7f94ce00a14f3d825a067a807eb12
Closes-Bug: #1779606
54d3e7096c