[Feature] auto_delete_orphans not support back_populates

[CroMarmot]

auto_delete_orphans seems only support backref, not support back_populates

    auto_delete_orphans(Magnet.tags)
  File "/usr/local/lib/python3.10/site-packages/sqlalchemy_utils/listeners.py", line 246, in auto_delete_orphans
    raise ImproperlyConfigured(
sqlalchemy_utils.exceptions.ImproperlyConfigured: The relationship argument given for auto_delete_orphans needs to have a backref relationship set.

however in 2.0 doc https://docs.sqlalchemy.org/en/20/orm/backref.html says

The relationship.backref considered legacy, and use of relationship.back_populates with explicit relationship() constructs should be preferred.

[jamesrusso]

Yes, just ran into this.

[jamesrusso]

Did a quick PR which seems to fix it for me #740, the tests could use some work to handle this scenario.