gh-148242: Partial restoration of the historical behavior of defaultdict, without losing thread safety

[Changaco]

This is another possible solution to #148242. It breaks some existing code, but not quietly, and it has the advantage of refining the design of the defaultdict class a little.

Changes to the Python stdlib:

• The defaultdict.__missing__ method is dropped, replaced by a new thread-safe defaultdict.__getitem__.
• The private importlib.metadata._collections.FreezableDefaultDict class is dropped. It could be fixed instead, but it looks fundamentally problematic to me, and unless I missed something it isn't actually needed, so I removed it.

Change to the private C API: the static dict_unhashable_type function becomes the exposed _Py_dict_unhashable_type.

Alternatives:

• keeping the defaultdict.__missing__ method (gh-148242: Full restoration of the historical behavior of defaultdict, without losing thread safety #148880)
• deprecating the defaultdict.__missing__ method
• dropping defaultdict's support for __missing__ methods entirely

• Issue: Behaviour change in defaultdict.__missing__ between 3.13.11 and 3.13.12 #148242

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