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

先重現 issue 中的 typing.List[int] 範例，並檢查泛型別名的實作與測試；未指定任何檔案或測試。首先確立任意 dunder 屬性與快取的 typing 物件的預期行為，然後加入回歸測試覆蓋，顯示行為一致，且屬性不會在物件之間洩漏。