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

mutagen/_util.py

@@ -522,7 +522,11 @@ def __eq__(self, other):
     def __lt__(self, other):
         return dict(self.items()) < other
 
-    __hash__ = object.__hash__
+    # DictMixin overrides __eq__ with content comparison, so objects with
+    # equal content but different identities would compare equal but hash
+    # differently when using object.__hash__. Since these tag objects are
+    # mutable, make them explicitly unhashable (like Python's built-in dict).
+    __hash__ = None  # type: ignore[assignment]
 
     def __len__(self):
         return len(self.keys())

tests/test__util.py

@@ -123,6 +123,12 @@ def test_repr(self):
     def test_len(self):
         self.failUnlessEqual(len(self.rdict), len(self.fdict))
 
+    def test_unhashable(self):
+        # DictMixin defines __eq__ by content, so objects with equal content
+        # would hash differently under object.__hash__ (identity).  The class
+        # must be unhashable to respect Python's hash/equality invariant.
+        self.failUnlessRaises(TypeError, hash, self.fdict)
+
     def tearDown(self):
         self.failUnlessEqual(self.fdict, self.rdict)
         self.failUnlessEqual(self.rdict, self.fdict)