fix(reassembly): Deferred pickle format/subclass gaps left by #2424 #2425

Describe the bug

Follow-ups surfaced during cross-review of PR #2424 (Deferred.__getstate__/__setstate__, fixing #2423). Neither is reachable in the repo today.

1. Pickle format changed for protocols 2+, not just 0/1. The new __getstate__ returns a dict built from __slots__; the old slot-aware __reduce_ex__ path returned a different shape. A Deferred pickled by any released version (v1.5.0b1–b14) fails to load under fix(reassembly): make Deferred picklable under protocol 0 and 1 #2424's fix with AttributeError: 'tuple' object has no attribute 'items', and the reverse direction fails too.
2. self.__slots__ is not MRO-aware. A hypothetical subclass of Deferred with its own __slots__, or one that adds a __dict__, would silently lose state under the new __getstate__/__setstate__ — the old __reduce_ex__ path handled both. No subclass exists in the repo currently.

Expected behavior

Either accept both the old (None, dict)-tuple shape and the new dict shape in __setstate__ (fixes 1), and make slot collection MRO-aware via itertools.chain.from_iterable(cls.__slots__ for cls in type(self).__mro__ if hasattr(cls, '__slots__')) (fixes 2) — or document that Deferred pickles are not stable across versions / across subclassing, if that's an acceptable limitation.

Additional context

Also flagged: the new test's docstring overclaims that a bound method is unpicklable under any protocol — IPv4.analyze pickles fine on 0–5. Worth a doc fix in whichever PR picks this up.