* tests/*.py: Do not prepend None to sys.path, Python 3.3 redeems that with an unintelligible crash. * tests/test_auth.py: In test_add_key_from_server_mitm(), show the exception if it does not match the expectation, so that this becomes possible to debug. * aptsources/distro.py: Replace the deprecated getiterator() ElementTree method with iter(), to avoid raising a PendingDeprecationWarning. * tests/test_auth.py: Temporarily disable $http_proxy for the tests, as gnupg does not get along with proxies (LP #789049)