_util: make DictMixin unhashable to fix hash/equality contract violation

[gaoflow]

Fixes #603.

Problem

DictMixin.__eq__ compares objects by their tag content:

def __eq__(self, other):
    return dict(self.items()) == other

But __hash__ was set to object.__hash__, which is identity-based. This violates Python's fundamental requirement that objects which compare equal must have the same hash value:

from mutagen.flac import FLAC
f1 = FLAC("test.flac")
f2 = FLAC("test.flac")
assert f1 == f2          # True — same tags
assert hash(f1) == hash(f2)  # False — broken!

Any code that puts FLAC (or other tag) objects into sets, uses them as dict keys, or relies on the hash/equality contract is silently broken.

Fix

Set __hash__ = None in DictMixin, making instances explicitly unhashable — consistent with Python's own mutable dict. All DictMixin subclasses (tag types, file types) are mutable mappings, so unhashability is the correct behavior.

Testing

Added a regression test in TDictMixin.test_unhashable. Full suite (3993 tests) passes.

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This pull request was prepared with the assistance of AI, under my direction and review.

[phw]

Right. the DictMixin must not be hashable. The docs say explicitly:

If a class defines mutable objects and implements an eq() method, it should not implement hash(), since the implementation of hashable collections requires that a key’s hash value is immutable (if the object’s hash value changes, it will be in the wrong hash bucket).

There is some small potential of this breaking some code, though, if code relied on the hasability of the instances. But regardless, I think this should be fixed and we mention this change prominently in the changelog.

[lazka]

risky, but makes sense.

There are more places with the same pattern