TLDR; execute response.selector.remove_namespaces() before running response.xpath()
It essentially means that you are removing xmlns="http://www.w3.org/2005/Atom" from response to write easier XPath.
Alternative, you can register the namespace and change your selectors to include this namespace:
response.selector.register_namespace('n', 'http://www.w3.org/2005/Atom')
response.xpath('//n:entry')
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