> I run my test suite on 2.7, 3.4, 3.5, 3.6, 3.7, 3.8, PyPy2, 
> and PyPy3.  Today, one gold file suffices for all those
> versions.  Without the sorted attributes, 3.8 fails the test.

Would it make sense to rewrite the test to just make sure the XML roundtrips instead of relying on the exact byte sequence that is generated?  IIRC, that is how we test pickling and various codecs -- rather than relying on implement specific details for a particular byte sequence -- the test is roughly "assert decode(encode(thing)) == thing".  That allows the test to survive on-going improvements to serialization.