typing._GenericAlias objects allow arbitrary dunder attribute creation

Bug report

Bug description:

Objects like typing.List[int] allow arbitrary dunders to be set, and because of the cache, those attributes can leak to other objects created elsewhere in the program:

>>> x = List[int]
>>> x.__whatever__ = 42
>>> List[int].__whatever__
42

The cache behavior seems like it could lead to undesirable behavior. In 3.14 we somewhat accidentally fixed a similar bug with Union objects, and it turns out some people were relying on this: #132139. Let's think about whether we also want to remove this wart for other typing objects in 3.15.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Empieza reproduciendo el ejemplo typing.List[int] del issue e inspecciona la implementación y las pruebas de los alias genéricos; no se nombra ningún archivo ni prueba. Primero establece el comportamiento previsto para atributos dunder arbitrarios y objetos de typing almacenados en caché, y después añade cobertura de regresión que demuestre que el comportamiento es coherente sin que los atributos se filtren entre objetos.