Adaptation doesn't respect Liskov invariance

An object can't be adapted even if one of its superclasses can: this breaks the Liskov invariance as the subclass is expected to behave like the superclass. In practice, for instance, if s.b. defines a string subclass, it is likely he wants it to be serialized in the database as a string. Currently psycopg raises a "can't adapt" (at least now it tells the class, so debugging is easier).

Proposal: if an object b of type B can't be adapted (it would be an error right now), walk its type's MRO and, if an adapter f for a supertype A is found, register the same adapter f for B too. If possible, detect the scope in which the adapter is registered for A (global, connection, cursor) and register the f as adapter for B in the same scope.