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

まず issue の typing.List[int] の例を再現し、generic aliases の実装とテストを調べます。ファイルやテストの名前は指定されていません。最初に、任意の dunder 属性とキャッシュされた typing オブジェクトについて意図された動作を確立し、その後、オブジェクト間で属性が漏れず、動作が一貫していることを示す回帰テストを追加します。