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

首先复现 issue 中的 typing.List[int] 示例，并检查 generic aliases 的实现和测试；没有指定文件或测试。首先确定任意 dunder 属性和缓存的 typing 对象应有的行为，然后添加回归测试覆盖，以表明行为是一致的，且属性不会在对象之间泄漏。