testing: make Eq(nan,<meh>) return False, Ne(nan, <meh>) return True

Not necessarily intended for a merge...
Its really just `Eq(nan, nan) == True` and `Ne` that I object to in principle :)

IEEE 574 floating point has nan's mathematically comparable and not equal.  So I'm inclined to follow that.  OTOH, I suppose an error would be consistent with trapping nan as a floating point exception, so in this sense would not be not contradictory to IEEE 754 FP.

I will look into `_eval_Eq`.