typing updates to accept with_polymorphic(), aliases

typing updates to accept with_polymorphic(), aliases

Fixed typing issues where ORM mapped classes and aliased entities could not
be used as keys in result row mappings or as join targets in select
statements. Patterns such as ``row._mapping[User]``,
``row._mapping[aliased(User)]``, ``row._mapping[with_polymorphic(...)]``
(rejected by both mypy and Pylance), and ``.join(aliased(User))``
(rejected by Pylance) are documented and fully supported at runtime but
were previously rejected by type checkers. The type definitions for
:class:`._KeyType` and :class:`._FromClauseArgument` have been updated to
accept these ORM entity types.

Fixes: #13075
Change-Id: Icc3b1ef832b01fd205b1409b2f6d0f211395d4ad
(cherry picked from commit 48410f83407661e009326d7170ab79e5163eb8f1)