In FullLoader python/object/new constructor, implemented by
construct_python_object_apply, has support for setting the state of a
deserialized instance through the set_python_instance_state method.
After setting the state, some operations are performed on the instance
to complete its initialization, however it is possible for an attacker
to set the instance' state in such a way that arbitrary code is executed
by the FullLoader.

This patch tries to block such attacks in FullLoader by preventing
set_python_instance_state from setting arbitrary properties. It
implements a blacklist that includes `extend` method (called by
construct_python_object_apply) and all special methods (e.g. __set__,
__setitem__, etc.).

Users who need special attributes being set in the state of a
deserialized object can still do it through the UnsafeLoader, which
however should not be used on untrusted input. Additionally, they can
subclass FullLoader and redefine `get_state_keys_blacklist()` to
extend/replace the list of blacklisted keys, passing the subclassed
loader to yaml.load.