This fixes an error where there is a different result for if 2.0.5 is contained within >2.0dev and >2.0.dev even though normalization rules should have made them equal.
In Python 3.x a subclass will not inherent the __hash__ method from the parent classes if the subclass defines a __eq__ method. This means that without defining our own __hash__ the SetuptoolsVersion classes are unhashable.