Addons: don't return any version when project is "single version"

[humitos]

If you go to https://pycamp.es/ you will see the flyout shows two versions: latest and 2022. However, this project is marked as "Single version". If you click on them, they give you 404 URLs.

readthedocs.org/readthedocs/proxito/views/hosting.py

Lines 272 to 279 in 6816afe

	"versions": [
	{
	# TODO: name this field "display_name"
	"slug": version.slug,
	"url": f"/{project.language}/{version.slug}/",
	}
	for version in versions_active_built_not_hidden
	],