Comment 7 for bug 1824299

I've created https://github.com/sqlalchemy/sqlalchemy/issues/4686 to think about a possible mitigation here, however I haven't had time to consider both the full span of races that can occur if classes are used while imports happen, as well as if there are certain races that are much more likely and therefore worth mutexing. In particular, the test script I have on my github issue produces this error in a slightly different way, and already shows that the above mutex I have in relationship.mapper() is not enough, depending on where the class setup process stalls.