dont match partial types in type_annotation_map

dont match partial types in type_annotation_map

Fixed issue regarding ``Union`` types that would be present in the
:paramref:`_orm.registry.type_annotation_map` of a :class:`_orm.registry`
or declarative base class, where a ``Mapped[]`` element that included one
of the subtypes present in that ``Union`` would be matched to that entry,
potentially ignoring other entries that matched exactly.   The correct
behavior now takes place such that an entry should only match in
``type_annotation_map`` exactly, as a ``Union`` type is a self-contained
type. For example, an attribute with ``Mapped[float]`` would previously
match to a ``type_annotation_map`` entry ``Union[float, Decimal]``; this
will no longer match and will now only match to an entry that states
``float``. Pull request courtesy Frazer McLean.

Fixes #11370
Closes: #11942
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/11942
Pull-request-sha: 21a3d1971a04e117a557f6e6bac77bce9f6bb0a9

Change-Id: I3467be00f8fa8bd011dd4805a77a3b80ff74a215