gh-145119: Allow frozendict to be assigned to instance __dict__

[mohsinm-dev]

Allow frozendict (and its subclasses) to be assigned to an instance's __dict__, enabling immutable instances.

Changes:

• _PyObject_SetDict: accept frozendict via PyAnyDict_Check
• _PyDict_SetItem_LockHeld: raise TypeError on set/delete instead of SystemError
• object___dir___impl: convert frozendict to a mutable dict for merging

• Issue: Consider allowing frozendicts to be assigned to instance dicts #145119

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Avoid frozendict dict on objects mutating their own dict

Allowing _PyObject_SetDict() to accept PyAnyDict_Check here makes frozendict assignable to every PyObject_GenericSetDict user, but some of those types still perform internal writes with PyDict_SetItem/PyDict_DelItem (for example descriptor_get_wrapped_attribute and descriptor_set_wrapped_attribute in Objects/funcobject.c for classmethod/staticmethod annotation caching). PyDict_SetItem still rejects frozendict with PyErr_BadInternalCall, so after obj.__dict__ = frozendict(...), operations like reading or updating __annotations__ can now raise SystemError instead of behaving normally or reporting an intentional immutability error.

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