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Eric, yes the key code in test_xml_etree that handles this is:
 def pickleRoundTrip(self, obj, name, dumper, loader):
 save_m = sys.modules[name]
 try:
 sys.modules[name] = dumper
 temp = pickle.dumps(obj)
 sys.modules[name] = loader
 result = pickle.loads(temp)
 except pickle.PicklingError as pe:
 # pyET must be second, because pyET may be (equal to) ET.
 human = dict([(ET, "cET"), (pyET, "pyET")])
 raise support.TestFailed("Failed to round-trip %r from %r to %r"
 % (obj,
 human.get(dumper, dumper),
 human.get(loader, loader))) from pe
 finally:
 sys.modules[name] = save_m
 return result
Because pickle does its own import of ElementTree.
The test___all__ problem should be solved by a patch to issue #1674555, but having a separate mechanism in test.support was discussed. I'll take a look at your decorator.