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- Introduce new `RoutingSession` method `execute_with_bind()` to execute
statements with a specific bind.
- `RoutingSession.using_bind()` is now deprecated and kept for backward
compatibility.
Instead of a new session instance, it now returns a shim that maps a
minimal subset of session methods to the original session instance.
This change was necessary because the previous implementation of creating
a new session instance and copying all the original session's state
proved unstable when used with SQLAlchemy 1.4. (One reason being the new
autobegin feature of sessions changing interactions with `zope.sqlalchemy`.)