call super().__init_subclass__(); support GenericAlias

call super().__init_subclass__(); support GenericAlias

Improved the :class:`.DeclarativeBase` class so that when combined with
other mixins like :class:`.MappedAsDataclass`, the order of the classes may
be in either order.

Added support for mapped classes that are also ``Generic`` subclasses,
to be specified as a ``GenericAlias`` object (e.g. ``MyClass[str]``)
within statements and calls to :func:`_sa.inspect`.

Fixes: #8665
Change-Id: I03063a28b0438a44b9e028fd9d45e8ce08bd18c4