Visit a class node.

def visit_ClassDef(self, node):
        """Visit a class node."""
        self.defined_types.add(node.name)

        bases = []
        for n in node.bases:
            base_parts = []
            while True:
                if not isinstance(n, ast.Attribute):
                    base_parts.append(n.id)
                    break
                else:
                    base_parts.append(n.attr)
                n = n.value
            bases.append(".".join(base_parts[::-1]))
        self.required_types |= set(bases)

        signature = get_signature(node)
        stub_node = ClassNode(node.name, bases=bases, signature=signature)
        self._parents[-1].add_child(stub_node)

        self._parents.append(stub_node)
        self.generic_visit(node)
        del self._parents[-1]