After searching for https://stackoverflow.com/questions/tagged/python-requests+or+urllib2+or+urlib3 I got rss link https://stackoverflow.com/feeds/tag?tagnames=python-requests+or+urllib2+or+urlib3&sort=newest which does not point to a valid rss feed.
For some reason if one of the tags doesn't exist (in your url, that'd be
urlib3) then we 404 feeds. Now we'll chug along so long as at least one of the passed tags exist, which seems a lot more sensible.
This fix will go out with the next build.