Fix missing class attributes when using __class_getitem__

Fix missing class attributes when using __class_getitem__

Fixed issue where the ``__class_getitem__()`` method of the generated
declarative base class by :func:`_orm.as_declarative` would lead to
inaccessible class attributes such as ``__table__``, for cases where a
``Generic[T]`` style typing declaration were used in the class hierarchy.
This is in continuation from the basic addition of ``__class_getitem__()``
in :ticket:`7368`. Pull request courtesy Kai Mueller.

Fixes: #7462
Closes: #7470
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/7470
Pull-request-sha: d5e5765e0e5542149f116ed9ccff1b3e2e32dee5

Change-Id: I6418af6d34532ff181343884bd419d9c2684e617