Import all legacy classes by sqlalchemy.orm.collections.*
Restored legacy class names removed from
``sqlalalchemy.orm.collections.*``, including
:class:`_orm.MappedCollection`, :func:`_orm.mapped_collection`,
:func:`_orm.column_mapped_collection`,
:func:`_orm.attribute_mapped_collection`. Pull request courtesy Takashi
Kajinami.
Fixes: #11435
Closes: #11432
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/11432
Pull-request-sha:
66f20a8e2069f48665299d1ee220dfe57aedf79e
Change-Id: I05172669fc9a44e737b3714001d1317bbbf0012f