implement polymorphic_abstract=True feature

implement polymorphic_abstract=True feature

Added a new parameter to :class:`_orm.Mapper` called
:paramref:`_orm.Mapper.polymorphic_abstract`. The purpose of this directive
is so that the ORM will not consider the class to be instantiated or loaded
directly, only subclasses. The actual effect is that the
:class:`_orm.Mapper` will prevent direct instantiation of instances
of the class and will expect that the class does not have a distinct
polymorphic identity configured.

In practice, the class that is mapped with
:paramref:`_orm.Mapper.polymorphic_abstract` can be used as the target of a
:func:`_orm.relationship` as well as be used in queries; subclasses must of
course include polymorphic identities in their mappings.

The new parameter is automatically applied to classes that subclass
the :class:`.AbstractConcreteBase` class, as this class is not intended
to be instantiated.

Additionally, updated some areas of the single table inheritance
documentation to include mapped_column(nullable=False) for all
subclass-only columns; the mappings as given didn't work as the
columns were no longer nullable using Annotated Declarative Table
style.

Fixes: #9060
Change-Id: Ief0278e3945a33a6ff38ac14d39c38ce24910d7f