[mypyc] Call __delattr__ of a class that inherits __setattr__

[rheard]

Fixes mypyc/mypyc#1241

del obj.attr and delattr() go through tp_setattro. The wrapper for that slot was only generated for a class that defines __setattr__, and it calls __delattr__ only if that class has one. A subclass that defines __delattr__ and inherits __setattr__ also inherited the wrapper, so its __delattr__ was never called:

class Base:
    def __setattr__(self, name: str, value: object) -> None: ...

class Sub(Base):
    def __delattr__(self, name: str) -> None: ...  # not called by del or delattr()

Now a class that defines __delattr__ and inherits __setattr__ from a native class gets a wrapper of its own, which calls its __delattr__ and the inherited __setattr__. tp_setattro is filled for every class that has a wrapper, so the slot table is keyed on the wrapper instead of on __setattr__.

The errors for __setattr__ in a class that allows interpreted subclasses or inherits from a non-native class are still reported only where __setattr__ is defined.

A native class that inherits __setattr__ from a non-native class doesn't get a wrapper. The slot it inherits from the non-native class looks up both methods at runtime, so that case already worked.