doc: Remove errant newline in the plugin tutorial
doc: Remove errant newline in the plugin tutorial

{% include %} syntax cannot have a newline.

Closes-Bug: #1774054

This commit also cleans up several things.

* setup.cfg: Rename "classifiers" to the correct keyword "classifier"
* setup.cfg: Add Python 3 classifier
* Quote literals

Change-Id: I14c2217a69ea489c72b311b26b5eb4a5e4865059

openstack/horizon,ChameleonCloud/horizon,ChameleonCloud/horizon,NeCTAR-RC/horizon,openstack/horizon,NeCTAR-RC/horizon,openstack/horizon,openstack/horizon,ChameleonCloud/horizon,NeCTAR-RC/horizon,ChameleonCloud/horizon,NeCTAR-RC/horizon