This fixes a regression in commit 60293416db that
changed the `__hash__` implementation of Node from the default pointer
hash, to a hash based on the node fields.
Since these fields contains list objects, they are not hashable, making
every call to `Node.__hash__` fail.
This breaks some third-party usage such as in `django-compressor`
(See: https://github.com/django-compressor/django-compressor/issues/1060)
This changed reverts the hash method back to using `object.__hash__` as
the hash implementation.
