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CPython expects __builtins__ to be a dict, but it is interesting to be able to use another type. For example, my pysandbox project (sandbox to secure Python) requires a read-only mapping for __builtins__.
The PEP 416 was rejected, so there is no builtin frozendict type, but it looks like the dictproxy type will be exposed as a public type.
Attached patch uses PyDict_CheckExact() to check if __builtins__ is a dict and add a "slow-path" for other types. The overhead on runtime performance should be very low (near zero), PyDict_CheckExact() just dereference a pointer (to read the object type) and compare two pointers.
The patch depends on issue #14383 patch (identifier.patch) for the __build_class__ identifier. I can write a new patch without this dependency if needed.