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

[Changaco]

This is one of the possible solutions to #148242. It has the advantage of being transparent to users since it doesn't break any existing code. The downside is that it doesn't completely fix the unsafe design of the defaultdict class.

Changes to the private C API (not strictly necessary, could be dropped):

• the static dict_unhashable_type function becomes the exposed _Py_dict_unhashable_type
• a _PyCriticalSection_WarnIfNotHeld function is created

Minor fix of the defaultdict.__missing__ docstring.

Alternatives:

• deprecating the defaultdict.__missing__ method
• dropping the built-in defaultdict.__missing__ method while keeping support for custom __missing__ methods (gh-148242: Partial restoration of the historical behavior of defaultdict, without losing thread safety #148881)
• 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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📚 Documentation preview 📚: https://cpython-previews--148880.org.readthedocs.build/

[python-cla-bot]

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