Add __class_getitem__ to the declarative Base class

Add __class_getitem__ to the declarative Base class

Fixed issue where the :func:`_orm.as_declarative` decorator and similar
functions used to generate the declarative base class would not copy the
``__class_getitem__()`` method from a given superclass, which prevented the
use of pep-484 generics in conjunction with the ``Base`` class. Pull
request courtesy Kai Mueller.

Fixes: #7368
Closes: #7381
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/7381
Pull-request-sha: 7db7fd869a6bb617f767fad5b71ddf7cb5f14ff5

Change-Id: I495718c3082ad6dd2c83fdbf6feba7c529e351cb