gh-89828: Do not relay the __class__ attribute in GenericAlias (GH-93754)

list[int].__class__ returned type, and isinstance(list[int], type)
returned True. It caused numerous problems in code that checks
isinstance(x, type).
(cherry picked from commit f9433fff476aa13af9cb314fcc6962055faa4085)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>