[mypyc] Call subclass __setattr__ for assignments through a base class

[rheard]

Fixes mypyc/mypyc#1242

An assignment to an attribute of a native class that has no __setattr__ is compiled to a direct write. The object can be an instance of a subclass that defines __setattr__, and then the method wasn't called:

class Base:
    def __init__(self) -> None:
        self.y = 0  # didn't call Derived.__setattr__

class Derived(Base):
    def __setattr__(self, name: str, value: object) -> None:
        print("setattr", name, value)
        super().__setattr__(name, value)

def f(b: Base) -> None:
    b.y = 1  # didn't call it either

f(Derived())

Now, if a subclass of the class defines __setattr__, the assignment checks whether the type of the object overrides attribute assignment (tp_setattro). If it does, the value is set with PyObject_SetAttr, which calls __setattr__. Otherwise the attribute is written directly as before. Nothing changes for classes that have no subclass with __setattr__.

I used a runtime check instead of always calling PyObject_SetAttr, because that would make every such assignment 10 to 20 times slower for objects that have no __setattr__ (about 2 ns to 26-47 ns per assignment in a loop). With the check, the difference for those objects is under 0.5 ns per assignment.