Any attempt to pickle an Element object in test_xml_etree.py causes that test to fail if test___all__ had been run before it; see attached transcript. It's against 3.4, with no changes other than the testsuite changes shown within.
I don't know what the root cause is. As noted before, adding 'xml.etree' to test___all__.AllTest.test_all.blacklist is a workaround, so I assume the root cause lies within the test framework --- a bad interaction between the import magics in test___all__ and test_xml_etree.