`Model.__eq__` treats distinct unsaved composite-PK instances as equal

[Denny Biasiolli]

Model.__eq__ still uses pk is None to decide whether an instance is unsaved. That is wrong for composite primary keys (pk is a tuple) and for db_default PKs (DatabaseDefault is not None).

>>> User() == User()
True
>>> User(tenant_id=1) == User(tenant_id=1)
True

Documented contract (and simple-PK behavior since #18864 / #18250) is that unsaved instances are equal only to themselves. __hash__ already uses _is_pk_set() and raises for an unset PK.

The XML serializer has the same leftover (if obj.pk is not None) and emits pk='["None", "None"]' for an unsaved composite instance instead of omitting the attribute.

Proposed fix: use _is_pk_set() in both places, matching __hash__ and the rest of the #373 conversion.

Related: #18864, #18250, #373.