Get rid of the "flipping behaviour" in narrowign by equality/identity

We have this code that makes sure that if the intersection of declared and narrowed is not representable, then we prefer the narrowed. I added this logic a while ago, since this is what a user usually expects in case of e.g. an assignment or even an isinstance() check.

However, the recent effort to make equality/identity narrowing symmetric also caused this to result in a "flipping" behavior, when sides exchange types after a comparison, see e.g. testNarrowGenericCallableEquality. Although such cases are rare, I think the resulting behavior is really weird/confusing.

So I propose to add a flag to each restriction in binder (similar to from_assignment), say prefer_declared that would be set to True for right-most operand in equality/identity. This flag would override the fallback mentioned above. So that after something like assert x == y == a, the type of a will not change if the intersection is not representable.

I understand this breaks the symmetry, but this is purely practical thing, this is why people write x == 2 and not 2 == x.

cc @hauntsaninja

Empieza con la lógica de fallback en mypy/checkexpr.py alrededor de las líneas 6494-6499 y luego sigue cómo se representan y aplican las restricciones en el binder. Ejecuta el caso testNarrowGenericCallableEquality para reproducir el comportamiento de inversión e inspecciona las pruebas cercanas de narrowing de igualdad/identidad. Se considera terminado cuando el operando situado más a la derecha conserva su tipo declarado cuando la intersección no se puede representar, sin cambiar el comportamiento habitual de narrowing.