When you look at the HTML source in a browser, you see that the feed XML is in a default namespace:
<feed xmlns="http://www.w3.org/2005/Atom">
All descendant elements of feed
also belong to this namespace - which is why your selectors do not yield anything. Except for the one selecting an attribute:
It seems only attributes such as @href are accessable
since attributes do not take on a default namespace - and remain in no namespace.
If you'd like to accesss elements that are in a namespace, you have to register the said namespace first, and choose a prefix for it:
xxs.register_namespace("atom", "http://www.w3.org/2005/Atom")
Then, prefix the elements with atom:
(or any other prefix):
xxs.select("//atom:link").extract()
Find more information in the relevant section of the Scrapy documentation.