sourceforge: retry for all retryable exceptions

Since this lister is doing a lot more requests than most other, it makes
sense that issues would arise more often. We want the lister to continue
even if the website is having issues and not break on the first 500 or
closed connection it encounters.

This change introduces a mechanism to retry all exceptions worth
retrying and uses it for the SourceForge lister. Other listers might
benefit from this, but this is out of scope here.

Tests had to be adjusted to stub the sleep function since retries happened
way more often.